'Nadal's 10 French Opens better than Federer's 18 slams'

Tennis expert Brad Gilbert has made a huge call on Rafael Nadal's 10 French Open titles in comparison to Roger Federer's 18 grand slams.

Nadal completed his 'La Decima' on Sunday with a straight sets victory over Stan Wawrinka in the French Open final, making him the first man in history to win 10 titles at the same grand slam.

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The 10th title at Roland Garros took Nadal's grand slam total to 15, passing Pete Sampras and closing the gap on Federer.

American Gilbert, a former player and highly-regarded coach, was asked on Twitter whether Nadal's 10 French Open titles actually outweigh Federer's record grand slam haul of 18.

"Both outrageous but probably 10 Roland Garros titles," he replied.

The huge call will undoubtedly cause some controversy among Federer's fan base, with debate continuing to rage about who is the greatest player of all time.

Fellow American Andre Agassi also commented on the Federer-Nadal debate recently, saying the Swiss great's 2017 Australian Open victory is 'harder to comprehend' than Nadal's 'La Decima'.

"This is one of those scenarios where the statistics alone put Nadal's French Open achievements into perspective," the eight-time major winner said.

"I think the harder thing to put into perspective is what Federer accomplished in Australia.

"I mean six months away from the game, the competition and at his age (35) and he goes out there and wins the way he did with multiple five-setters.

"That's a little bit harder to tangibleise (sic) but winning 10 times - winning nine times, forget about 10 times - all you have to do is write that down on a piece of paper.

"It puts into perspective."

The great rivals will lock horns again when Wimbledon starts on July 3.

Federer is tuning up at the Mercedes Cup in Stuttgart after a two-month absence from the tour, while Nadal has withdrawn from Queen's to rest his knees for a shot at the All England Club title.