Athleticism and tennis. When combined they make a grand show for tennis fans.

One witnessed it in the 2012 Australian Open final when the two top seeds, Novak Djokovic and Rafael Nadal, played a marathon final for nearly six hours—the longest ever by a wide margin.

With the recent advances in fitness levels, players have become more athletic than ever before. But one cannot discount players from previous generations who were fit enough to challenge the current crop.

We do not have the luxury of comparing players from different generations, and let us not go anywhere close to doing such an activity.

The focus of this article is on 20 most athletic players. Who are they? 

Rafael Nadal might have lost the marathon final against Novak Djokovic in Australia, but that does not deter him from claiming the most athletic tennis player of all time.

Nadal's success on clay is proof enough of his grit and endurance. Arguably the best ever clay-court player in the world, Nadal has to be on the top of the throne in this category. 

There has been no one (until Novak Djokovic came along) who could match Nadal's returns and ground him down on court when he is on song. 

Experts are surprised that Nadal is still playing without any major injuries, considering the workload he has carried so far in his career.

Hopefully, he can continue delighting his fans.

In 2003, at the age of 46, Martina Navratilova won the Mixed Doubles at Wimbledon!

Just a month away from her 50th birthday, Martina bowed out of the Grand Slam with a mixed doubles loss.

She is often called the greatest all-time player in the history of tennis and will remain forever in our hearts—Martina is the best!

The following statistical nuggets say a lot about the Great Legend.

• Only male player in tennis history to have reached the title match of each Grand Slam tournament at least five times
• Holds the record of reaching the semifinals or better of 23 consecutive Grand Slam tournaments over five and a half years
• Reached a record 31st consecutive Grand Slam quarterfinal at 2012 Australian Open

Do we have any further doubts? 

The current No. 1 on ATP Tour, Novak Djokovic, has all the weapons he needs to beat his opponents. 

He also has good legs to prop him up and keep him on the court for long periods.

Though an all-court player, Djokovic has the ability to imitate Nadal when required (run around the baseline to hit winners repeatedly).

Michael Chang's name brings back memories of a player who used to run around all over the court and retrieve balls that were impossible (an early version of Rafael Nadal).

Though Chang won only one Grand Slam, he was famous for his never-say-die spirit and his fitness levels on court. 

One could not be sure of winning a point until Chang decided to give it away himself—such was his spirit and athleticism.

What more can you say about John McEnroe? 

A singles win-loss record of 875-198 along with a doubles record of 530-103 shows how fit the legend was.

John has played a couple of Davis Cup matches that were more than six hours.

• six-hour, 22-minute victory over Mats Wilander in a quarterfinal win over Sweden in 1982 by 9-7, 6-2, 15-17, 3-6, 8-6
• six-hour, 20-minute loss to Boris Becker in 1987 by 4-6, 15-13, 8-10, 6-2, 6-2

McEnroe is a strong athlete and tennis artist.

Steffi Graf won 22 Grand Slam singles titles, second among male and female players only to Margaret Court.

In 1988, Graf became the first and only tennis player to achieve the Calendar Year Golden Slam by winning all four Grand Slam singles titles and the Olympic gold medal in the same calendar year.

Steffi Graf was ranked No. 1 by the WTA for a record 377 total weeks—the longest period for any player.

The current coach of Andy Murray, Ivan Lendl was the scourge of many players in his time due to his running shots and top-spin shots.

Lendl's success in the game was mainly due to his training schedules and physical conditioning.

Pete Sampras spent 286 weeks at the top of the ATP Tour as No. 1—a testimony to his skills as well as his fitness.

Despite feeling sick (and vomiting on the court) in his quarterfinal match in the 1996 US Open against Alex Corretja, Sampras managed to pull off the match and eventually win the title.

Serena Williams' game is built around powerful serves (some of the fastest serves on WTA are recorded in her name).

With her strength and pin-point precision, Serena is the favorite of any tournament she enters (provided she can retain her temper).

Mats Wilander was another busy player on the court in his time—like John McEnroe.

Playing doubles with the same intensity as singles, Wilander was always on the ball.

Wilander (and Nadal) are the only men in tennis history to have won at least two Grand Slam singles titles on each of the three surfaces—a statistic that reflects his strengths.

Stefan Edberg was also active just like his fellow Swede, Wilander, in all formats of the game.

A thorough gentleman, Edberg still holds the record (with Michael Chang) for the longest game played in the US Open in 1992.

Venus Williams was the dominant force on the WTA tour until she met her match—sister Serena.

Venus Williams still holds the record for the fastest serve by a woman.

If not for injuries, the world would have witnessed more of this explosive player.

Andy Roddick's name brings back memories of his fast serves; he was a No. 1 just a few years ago.

The records that are still attributed to him show the man's strength—chances are that most of the Grand Slam tournaments still have his serve as the fastest ever in tournament records.

One hopes he returns to past glory—will he?

Andre Agassi continually put pressure on opponents with a preference for taking the ball early and swinging deep angles.

Known for his service returns, Agassi achieved so much in his career (thanks in part to his coach Brad Gilbert) with a zeal that few could match.

Arantxa Sanchez Vicario was the female Michael Chang on tour, known for her business on the court.

Sanchez was determined to follow every ball and return it to her opponents to demoralize them.

Most of the time it worked!

A former world No. 1, Yevgeny Kafelnikov was one of the few Russians who made it big on the ATP Tour.

One of the longest matches in the Davis Cup involved him and Marat Safin (in doubles) against the Argentinian team—a match that went on for more than six hours.

If anyone could match the Williams sisters in terms of speed and precise serves, it was Lindsay Davenport.

She overcame her slowness on court and mental fragility to fight back against her opponents and win more Grand Slams.

There will be no article on athleticism and endurance complete without the mention of Nicholas Mahut and John Isner. 

Their first-round match during 2010 Wimbledon went on for a whopping 11 hours and five minutes! A record that will stay for a long time to come (in the age of tie-breaks).

Mahut had to qualify for the main draw by playing two matches—one was a 24-22 set while the other match went for five sets.

Though they are not so highly placed at this moment, the record is a tribute to their fitness standards.