(CNN) Shortly after winning his 101st tournament on the hard courts of the Miami Open in April, Roger Federer cracked a joke about playing on clay.

"I didn't even remember how to slide anymore," said the Swiss superstar, when asked about the prospect of playing on the slow, red dirt in Europe this spring for the first time in three years.

With its high bounces and often long and grueling rallies, clay-court tennis can be an added grind on the body.

It is also the surface that least helps Federer's particular brand of attacking, first-strike tennis, even if has won 11 titles on clay, including the French Open in 2009.

Federer last played at Roland Garros in 2015, when he lost in the quarterfinals to fellow Swiss and eventual champion Stan Wawrinka.

But on his clay-court comeback at the Madrid Open Tuesday, he impressed with a 6-2, 6-3 win against Frenchman Richard Gasquet.

Surprise return

Federer's clay-court retirement began after an early loss in Rome in May 2016, and he pulled out of the French Open a few weeks later, saying he was not 100 percent fit.

He missed the second half of the season to recover from a knee injury, having already had arthroscopic left knee surgery earlier the same year for a torn meniscus. In the past two years, he has given the clay a miss to focus on Wimbledon.

But the 20-time Grand Slam champion is still firing on all cylinders, and keen to have "no regrets," he opted to make a return to the clay.

Boosted by his 100th Tour title in Dubai in February, a runner-up spot in Indian Wells and victory in Miami, he approached he clay-court swing in confident mood.

Roger Federer claimed his 100th career title and eighth Dubai crown with a victory over Stefanos Tsitsipas of Greece.

'More stress'

At 37, though, Federer's return to the surface he grew up on in Switzerland was still somewhat of a surprise.

Pierre Paganini, Federer's long-time physical trainer, had been apprehensive about the former top-ranked Swiss going back on the clay, telling the New York Times in 2017: "The disadvantage with the slide on clay is that in the joints there is a lot of vibration. We don't see it from the outside, but to control this slide there is instability in the knee, the foot, the ankle. And that in some cases can be bad for the knee or joint in question."

Playing on clay, particularly in damp conditions, "places more stress on the player's cardiovascular system and also muscular endurance," because the balls become heavier, and the rallies longer, Ian Prangley, physiotherapist and physical coach for British No. 1 Kyle Edmund, told CNN Sport.

"If a player hasn't maintained their aerobic fitness and endurance capacity during the hard court season, then that transition to longer clay-court points can be a problem," added Prangley.

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'Ready for long rallies'

Ahead of his opening match in Madrid, Federer said: "You have to put in the hours on clay again.

"You have to get used to sliding. It's about the different pressures that clay puts on your thigh and calf muscles compared with grass, for example. When you're playing on grass it's more about explosivity.

"Here, it's a bit more about endurance. You just have to work on that. The work is difficult and it's hard. You have to be ready for long rallies and long matches, so you have to work on different things for your endurance."

Federer made the decision to return to clay last December, he said, but only started practising on the surface at home in Switzerland last month, training at altitude to get used to playing in Madrid, which has an elevation of 650 meters.

The knee, he said, is no longer on his mind. "I feel very strong right now," he said in Madrid. "Obviously we worked on that in December already. So the knee has been long gone, to be honest. That was all done by mid-17, I'd say, after I won Wimbledon."

Prangley said the five-week preparation to the clay-court season will have allowed Federer to get his body in shape for the particular rigors of the surface, adding that he would have been working on his aerobic and endurance base, and strength.

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'Efficient mover'

Throughout his long career, Federer has remained relatively injury free, especially compared to his rivals Nadal, top-ranked Novak Djokovic and Andy Murray.

"Roger is a very efficient mover on a tennis court, and those players who are most technically proficient and therefore efficient when moving and hitting a tennis ball are at a lower risk of injury," said Prangley.

"This only comes with great technical training from excellent coaches, particularly when developing as a junior."

Federer's clever scheduling of tournament blocks, followed by time off for rest and recovery, will also help him in his clay comeback.

"Remaining injury free is knowing when to train and when to rest, so that your body can recover," said Prangley. "When you have a good team of coaches, trainers and physios that work together well, they can plan out a training and competition schedule to maximize performance while reducing injury risk."

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Nadal re-match?

For years, Federer was one of the best clay-court players in the world, second only to Nadal, the winner of 11 French Open titles against whom he is 0-5 in Paris.

The pair last met at Roland Garros in the 2011 final, and despite losing four title matches to the Spaniard, Federer would like nothing more than playing his old rival once more in the French capital.

"I would love to play on clay against him again, even though I know it's a tough challenge and all that," said Federer, who first played Nadal at Roland Garros in the 2005 semi-finals.

"But again it would be nice to have played him at the beginning of his the career on clay and also at the very end and see how it all plays out.

"And if I said I don't want to play him on clay, I think then I would have made a mistake to be on the clay in the first place because he is the measuring stick for all us players."