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.