Serena Williams has declared Roger Federer the greatest tennis player of all time, snubbing his great rivals Rafael Nadal and Novak Djokovic.
While fans and pundits continue to debate who should be considered the GOAT, 23-time grand slam winner Williams has no doubts.
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Nadal picked up a record-extending 13th French Open title last October to equal Federer's all-time record of 20 grand slam titles, while World No.1 Djokovic - the youngest of the 'Big Three' at 33 - is on 18.
On Tuesday, the 39-year-old Federer played just his second tournament since returning to the ATP Tour in March, after spending more than a year on the sidelines due to two knee operations.
"I think two words sum it up: Roger Federer," Williams, who turns 40 in September, told reporters at the Emilia-Romagna Open in Parma on Monday.
"He's just a synopsis of greatness and class and amazing and really changed the game.
"You see players playing like him, moving like him, doing his techniques. The guy is (a) genius."
Serena Williams heaps praise on Roger Federer
Williams won the last of her majors at the 2017 Australian Open before becoming a mother.
She has since been chasing a 24th major title to equal Margaret Court's record.
On Monday, the American won her first match since losing in the Melbourne semi-finals in February and said she was a Federer "super-fan".
"I just feel like he is really the greatest player," Williams said.
"You can't not like the guy, that's how I feel.
"His game is so fantastic. If I could only play like him."
Top seeded Federer began his campaign in Geneva on Tuesday, suffering a shock loss to Spaniard Pablo Andujar.
"It was something that I wanted, to have a match against Roger, just to tell my kids and my grandkids when I get older that I played him," Andujar told reporters.
"I think when you play Rafa, Novak and Roger ... these guys became myths, (they are) something above the sport.
"I would say that the most important is that they grew the sport."
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