Boasting a record number of grand slam wins, Roger Federer is considered by many to be the greatest tennis player of all-time.
However, retired American pro Andre Agassi - who is Novak Djokovic's coach - has explained why he does not agree.
Agassi won nine grand slams and was one of the sport's most dominant players from the early 1990s to the mid-2000s.
Rafael Nadal has won 16 to Federer's 19 grand slams - they've both won more than any other male in the history of the sport.
Nadal, who won the French and the US Open in 2017, could still conceivably catch Federer's incredible haul of 19.
Currently No.1 in the world, the Spaniard is five years younger than his rival but has had to battle persistent injury problems over the last few years.
Djokovic and Andy Murray have gone through a difficult spell but tennis fans have enjoyed the spectacle of Federer and Nadal returning to their former status as the best two players in the world.
Why Federer is not the GOAT
Agassi told French media outlet Nice Matin that he believes Nadal, 31, is superior to Federer.
The 36-year-old is often labelled as the 'Greatest of all-time' [GOAT] but Agassi has revealed why he believes Federer does not deserve this title.
"A player can not be considered the best of all time if he has been beaten so many times by an opponent," said Agassi referring to Federer's record against Nadal.
Despite winning all five of their most recent meetings, the veteran Swiss has won only 15 of their 38 encounters.
"Federer is unique," Agassi continued. "In all: greatness, accuracy, ease. He plays tennis so 'differently' from others.
"But Federer gives off that feeling of control, of having a solution for every problem. We can not deny that Federer will mark sport forever."
The duo are currently competing in the end-of-season ATP World Tour Finals in London.
Federer beat Jack Sock in straight sets on Sunday whilst Nadal is due to face David Goffin on Monday night.
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