(CNN) If you were a 37-year-old at the French Open Sunday, it was a day to remember.

Roger Federer made a winning return to the clay-court grand slam after a three-year absence while Nicolas Mahut added to his noteworthy accomplishments after losing the longest tennis match in history, overcoming a two-set deficit to defeat last year's surprise semifinalist Marco Cecchinato on the new Simonne Mathieu court.

Defeat to John Isner at Wimbledon in 2010 wasn't the end of Mahut, rather it served as the turning point in the Frenchman's inspiring career.

Federer's own career, one in which he leads the men's grand slam count on 20 and remains a viable threat at majors despite his advanced tennis years, has been inspiring too.

That is, of course, a huge understatement.

Playing for three decades -- he will probably reach a fourth -- Federer first began as the young, temperamental upstart.

He soon became the most dominant player in tennis, later overtaken by Rafael Nadal and Novak Djokovic and subsequently overcame a loss of form and injury to engineer one of the most dramatic comebacks in sport at the 2017 Australian Open.

Being, thus, accustomed to almost every scenario possible in sporting terms, Federer found himself in rather unfamiliar territory Sunday in coming back to a major following a lengthy, lengthy pause.

Photos: Federer makes winning French Open return Roger Federer acknowledges the crowd after returning, and winning, at the French Open. Hide Caption 1 of 10 Photos: Federer makes winning French Open return Not surprisingly there was a packed house for Federer, who hadn't played at the event since 2015. Hide Caption 2 of 10 Photos: Federer makes winning French Open return Federer swept past Italian Lorenzo Sonego in one hour, 41 minutes and will face a lucky loser in round two. Hide Caption 3 of 10 Photos: Federer makes winning French Open return Federer signed autographs and posed for a selfie or two after the win. Hide Caption 4 of 10 Photos: Federer makes winning French Open return It was a good day too for Federer's fellow 37-year-old Nicolas Mahut, who pulled off a stunning comeback. Hide Caption 5 of 10 Photos: Federer makes winning French Open return The French wildcard upset 2018 semifinalist Marco Cecchinato from two sets down, then celebrated with his son on court. Hide Caption 6 of 10 Photos: Federer makes winning French Open return Venus Williams, 38, got a tough draw and fell in straight sets to ninth seed Elina Svitolina 6-3 6-3. Hide Caption 7 of 10 Photos: Federer makes winning French Open return Three-time grand slam champion Angelique Kerber will have to wait to complete her collection of majors. Just back from an injury, she fell to former junior No. 1 Anastasia Potapova. Hide Caption 8 of 10 Photos: Federer makes winning French Open return Garbine Muguruza, a former French Open champion whose ranking has dropped to 19th, rallied from a set down to beat American Taylor Townsend. Hide Caption 9 of 10 Photos: Federer makes winning French Open return Greece's Stefanos Tsitsipas, having a breakout 2019 campaign, advanced in straight sets. Tsitsipas beat King of Clay Rafael Nadal recently in Madrid. Hide Caption 10 of 10

He decided to skip the clay-court swing the past two seasons to preserve his body and ready himself for Wimbledon, his most successful grand slam.

Away for a while

Since his last appearance in a Parisian spring in 2015 -- the banged up Swiss withdrew on the eve of the 2016 edition, ending a run of 65 straight majors contested -- the French Open has decided to undergo a much needed refurbishment.

The seat colors, for example, on the main Philippe-Chatrier court changed from green to grey and come next season a roof will finally be in place to match the three other majors.

Federer's French fans must have had a lump in their throats when he withdrew mid-tournament from the Italian Open a week and a half ago with a leg complaint but there he was Sunday looking completely at ease, dispatching Lorenzo Sonego 6-2 6-4 6-4.

Federer admitted he had more nerves than usual but was eased by the predictably warm welcome.

"The reception I got today was crazy, was really nice to see a full stadium for a first round like this," Federer told reporters. "It was a beauty. So I'm very, very happy.

"I feel that the public missed me, and I missed them, as well.

"So 10 or 20 years later or after not playing here for many years, there was some buzz, which I could feel on the central court when I was training and when I was playing today," Federer, who made his French Open debut in 1999 and won his lone Roland Garros title in 2009, later said in French. "So it was rather cool, rather pleasant, and I really loved the welcome I got on the court. I hope that it continues like this."

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Besides being multilingual, Federer also seems like a kidder. There is no chance of the French fans' love for him waning, including in the second round when he meets a German lucky loser, Oscar Otte.

Sonego may not be a household name to casual fans but the Italian is no slouch on clay having reached the quarterfinals at the Monte Carlo Masters in April.

But, in a sight witnessed dozens and dozens of times over the years, Federer won the first set early at a grand slam in the blink of an eye to quash hopes of an upset.

Sublime shot

His all-around game pleased the spectators, with perhaps the shot of the affair Federer executing a sublime drop-shot return winner to break in the second set.

3 years, 11 months and 25 days later...



Good to have you back!#RG19 pic.twitter.com/6PNYcrBBo5 — Roland-Garros (@rolandgarros) May 26, 2019

If Federer is to do the almost unthinkable at this stage of his career and win a second French Open title -- Nadal, Djokovic and Dominic Thiem are the top contenders -- he'll have to breeze through the first week.

Only spending one hour, 41 minutes on court made for a perfect start.

"No problems with my body before or after the match," said Federer, in the same half of the draw as French Open king Nadal. "It's ideal for me for the start of the tournament."

Federer admitted recently that his children enjoyed watching Sonego's fellow Italian, Cecchinato.

They will have to find another player to root for -- besides, obviously, dad -- after Mahut's sensational comeback.

Mahut, like Federer, particularly enjoys playing on grass, despite that 70-68 fifth-set reverse to Isner.

And when he dropped the first two sets to Cecchinato, a third consecutive first-round exit at the French Open appeared a certainty. Even more so since he had been dealing with a back injury.

Yet the 253rd-ranked Mahut weathered three break points at 3-3 in the fifth set -- a game in which he struggled to find first serves -- and broke to end proceedings for a second career victory from two sets down on the court surrounded by tropical plants.

He clearly justified his wildcard.

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Mahut's improbable win

Sunday's result, winning his first singles titles, achieving a career high in the rankings, representing France in the Davis Cup and completing the grand slam in doubles, have all come post the Isner heartbreak when aged 28.

"It is by far my best victory in Roland Garros, in a French tournament, I hadn't had many," said Mahut. "And playing on this extraordinary court, I felt that if there was a possibility of playing on this court, it would be incredible and there would be an incredible atmosphere.

"This victory will rank very high in my memories."

Federer has had greater wins, but he surely will be remembering his own experience Sunday with fondness.