Roger Federer. Rafael Nadal. Novak Djokovic. Andy Murray. These four tennis superstars, collectively known as the Big Four, have ruled tennis for the past decade, with each of them taking a turn at the top. In total, they have won 48 out of the last 56 Grand Slam dating back to Federer’s first Wimbledon in 2003. . For the past 14 years, they have dominated almost every tournament and produced one of the biggest rivalries of all time: Federer vs. Nadal.

This past year has been a throwback to the years of Federer-Nadal dominance. Though both players appeared to be declining veterans, Federer won this year’s Australian Open and Nadal won the French Open last month. Though the Big Four are now all over the age of 30, they do not seem to be slowing down in any way.. However, there will come a time in the (hopefully distant) future when they will all retire, relinquishing their hold on the tennis world. Injuries and lack of motivation might start to creep in as the years roll by, and eventually a new generation of tennis players will take over.

Some of these players have been labeled as the NextGen by many tennis experts and commentators because the Big Four’s dominance has stretched for so long.. These players are only 21 and younger, but have been marked as stars to watch in the future. Although the NextGen stars are considered exciting, there are some players in their mid-20s who could also possibly usurp the Big Four. With many new players starting to break onto the professional tennis scene, here are some of the players who will most likely bring the dawn of a new tennis era.

Alexander Zverev Jr.

Age: 20
Country: Germany
NextGen: Yes
Ranking: 12

Alexander “Sascha” Zverev is a tennis player from Germany who is considered one of the brightest NextGen tennis players. Tennis runs in his blood, as his parents both played tennis for the Soviet Union., His older brother, nicknamed “Mischa,” is also a professional tennis player and is ranked number 30 in the world. After turning professional in 2013, he has continuously been on the rise since proceeding to the semifinals of his first ATP tour event in 2014. In 2015, he broke into the top-100 and made it to his first Grand Slam competition at Wimbledon. He broke into the top-20 the following year. In 2016, he impressed many by competing with many of the top players, defeating his idol Roger Federer, and winning his first ATP title against 2016 US Open Champion Stan Wawrinka. . This year he made even bigger strides by breaking into the top-10 and winning his first ATP Masters tournament (a level below Grand Slams) against Novak Djokovic. His keys to success have been his strong, fast serve and his solid, dependable backhand.

Dominic Thiem

Age: 23
Country: Austria
NextGen: No
Ranking: 8

23-year-old Dominic Thiem of Austria has also been touted as one of the greatest players of this upcoming generation. After choosing to become a pro in 2011, he advanced to his first ATP final and the Fourth Round of the US Open in 2014. Since then, he has worked on his game to become a strong baseliner, using an aggressive one-handed backhand, which is rarely seen in someone so young. In 2015, despite not doing too well at the Grand Slams, he still consistently excelled on the ATP World Tour,, winning three ATP titles through his determination. By ascending in the rankings, Thiem entered a tournament as the top seed and finished the year ranked number 18.. Thiem translated his ATP Tour success to Grand Slams in 2016. A very strong clay court player, he upset then 9-time French Open champion, Rafael Nadal, en route to winning an ATP title on clay. At the French Open, he eventually reached the semifinals of a Grand Slam for the first time, which propelled him to number 7 in the world. In 2017, so far, he has won his eighth ATP title and advanced to his first Masters final. On the clay courts again, he recorded some major upsets by defeating Nadal at the the Rome Masters and demolishing Djokovic at the French Open in straight sets. His aggressiveness, consistency, and strong mentality has made him one of the greatest of his generation.

Milos Raonic

Age: 26
Country: Canadian
NextGen: No
Ranking: 7

The big-serving Canadian known for his philanthropic deeds is also one of the most promising players of his age. Despite being 26-years-old and turning pro nine years ago, he has come into his own in the last couple of years. The Big Four’s dominance overshadowed his achievements when he was in his early 20s, but he has been one of the few players who has slowly narrowed the gap and challenged the sport’s titans. Until 2010, Raonic mainly played in small ATP tournaments and qualified for the Grand Slams occasionally, but lost quite early. However, in 2011, he broke ground when he reached the Fourth Round of the Australian Open as a qualifier, the first to do so since 1999. Many former players praised Raonic, and he backed it up by winning his first ATP title to reach a career high of world number 25. In 2013, he consistently achieved success at many ATP tournaments and broke into the top-10 for the first time. In 2015, he reached the semifinals of a Grand Slam for the first time. Additionally, he reached the ATP World Tour Finals, a tournament where only the top eight players play, and was ranked as high as number four. Last year saw a new Raonic, as he was more consistent and determined than before. His hard work came to fruition when he reached the finals of Wimbledon, and ascended  to world number three. His strong forehand, serve and prowess on all surfaces has allowed him to grow into a strong player who could be the heir to the Big Four.

On August 23, 2003, one of the most legendary tennis players of all time bid farewell to the sport that he had given everything to and had given everything to him. Pete Sampras had officially announced his retirement from professional tennis after playing and winning his last match in the US Open finals a full year prior.

In a career spanning 15 years, Sampras broke many records, including winning the then most number of Grand Slams by a male in tennis history (14). The Grand Slams include the Australian Open, Wimbledon and the US Open, and are some of the most coveted tournaments in tennis. While many tennis greats struggled to win even one of them, Sampras excelled at almost all of them, save the French Open. He won two Australian Opens, a record-tying seven Wimbledon titles and five US Opens throughout his glimmering career to add to 50 other top-level titles. Along with his titles, he was also world number one for a then record number of 286 weeks, and was seeded number one at the end of six consecutive years, a record which showed his consistency when he was in his prime.

Sampras joined the professional tour in 1988 at the age of 16, and made an immediate impact throughout the year by beating many players in the top 100. He ended the year ranked 97 in the world, which was considered quite a feat considering that he started the year as world number 893. Throughout the next two years, he continued to challenge the top players by recording several major upsets. His time finally came in the 1990 US Open, where he won the first of his 14 Grand Slams. On his way to the finals, he registered major upset wins before going on to win the trophy in straight sets against his biggest would-be rival, Andre Agassi. Sampras was the US Open’s youngest-ever winner at 19 years, capping off his remarkable year at a career high of world number five.

For the next couple of years, he continued to play consistently, making deep runs in many tournaments, and although he did not win a Grand Slam, he found a new drive to become world number one. That moment arrived in 1993 when he became world number one in April. Sampras further supported his ranking by winning his first Wimbledon and second US Open later that year. As his career progressed, he started to win more titles consistently and battled for the number one spot with Agassi. Between 1994 and 1996, he won one Australian Open, two Wimbledon titles, two US Opens, along with making his deepest run in the French Open, where he lost in the semifinals to the eventual winner.

In 1997, Sampras won his second Australian Open and his fourth Wimbledon along with winning many of the biggest tournaments in the world. That year, he went undefeated in finals matches to hold the number one ranking at the end of the year for a record-tying five consecutive years. He would go on to extend that record to six next year and won another Wimbledon title that year despite a small dip in form. 1999 started out badly for Sampras as he could not win any singles titles during the first part of the season. However, he went on a massive 24-game winning streak afterwards, where he captured many titles, including his sixth Wimbledon. However, the year ended bleakly, just as it began, as he could not participate in the US Open and lost the number one ranking to Agassi.

The new millennium ushered in more record-breaking feats for Sampras, as he won the most number of Grand Slams singles titles, 13, and won his record-tying 7^th Wimbledon alongside that. He fought through back injury and tendinitis in the final to win in 4 sets, becoming the first person to win a Grand Slam for 8 consecutive years in the Open Era in the process. He returned to number one status that September, but could not stay there for long, which marked the last time he would be at the top of the rankings.

2001 was a completely different season than the prior year, as Sampras suffered many early exits in tournaments. In Wimbledon, where he thrived, he lost to the current grand slam record holder, Roger Federer, in the fourth round, which was then considered a major upset. He also lost in straight sets in the US Open final, among suffering other setbacks that signaled that his career would soon come to an end. In Sampras’ last year in which he played professionally, he lost to very low ranked players early in Wimbledon and other tournaments where he normally excelled in, leading many to believe he would not be able to win another Grand Slam. Ranked world number 17 going into the US Open, it seemed that it was finally time for other big names in tennis to take the spotlight. However, he turned things around in the tournament, progressing to the finals for a third straight year to poetically play Andre Agassi. The two rivals and fellow Americans gave the audience and fans at home a match for the ages, playing four sets with Sampras triumphing at the end to win his 14th title.

A year later, after playing his last match in the previous year’s US Open final, Sampras was given a tribute for his outstanding service to the game. As Sampras said himself, “people wrote me off, but I believed in myself. I got the confidence back, and it grew and grew. I won my first major and my last at the place that changed my life.” Pete Sampras will always be remembered for his contributions to modern tennis in an era when there were plenty of high-level players to overcome.