Rod Laver saluted Roger Federer's feat of winning a 15th Grand Slam as "an unbelievable effort".

But Australian Laver, who won two calendar Grand Slams during his distinguished career, stressed it was impossible to compare players from different eras and name the sport's greatest star.

Federer, 27, broke the record of 14 career Grand Slam titles, which he shared with Pete Sampras, to become the most successful player of all time.

Laver, now 70, said: "It's amazing what sorts of shots he can come up with from impossible positions.

"It's a great feeling to watch the talent he has and the opponents that he beats comfortably, where other players have such a tough time. With a player like (Ivo) Karlovic, the other players can't get his serves back but how come Roger can do it?

"He's just naturally talented and can change where he has to change.

"It's an unbelievable effort to have 15 Grand Slam titles.

"Pete Sampras has got 14, which was an unbelievable effort right there."

Laver won 11 Grand Slam titles but from 1963 to 1967, during the peak years of his career, he was prevented from playing at Wimbledon and the Australi, US and French Opens, which were only open to amateurs.

He won Wimbledon in 1961, 1962, 1968 and 1969, and would almost certainly have claimed many more slams, which would have meant Federer would now be chasing Laver's record rather than out in front by himself.

Laver duly claimed it was difficult to compare players from different eras.

He said: "I've always thought that if you're the best in your era that is a pretty good compliment to your game and your tennis over your career.

"It's hard for anyone to come out and say who's the best ever."

Laver nevertheless does not lack admiration for Federer's incredible record, which began with a first Wimbledon title in 2003.

"You almost think about table tennis when you think of the way Roger plays with a racquet," said Laver.

"The public should just watch his feet, just watch Roger and not the ball, and you'd see how great a player he is to pull off some of the shots.

"When he's half-volleying winners off the baseline, you just marvel at the ability to do that.

"To analyse his game is hard but he's got so many spins, he's co-ordinated and anticipates so well."

Laver believes Federer's friendship with golf world number one Tiger Woods has driven the man from Basle, Switzerland, to new heights.

"You've got to be in the game and enjoy the sport to be able to do something like this," continued Laver.

"You're not going to (win) 12 or 13 events if you don't respect the game and enjoy it. It's a thrill for you to get out there and play.

"That's the one thing Roger has, that I think is admirable for tennis.

"It's great that tennis has someone like Roger. He and Tiger Woods are good friends, fighting to see who can have the best number of Grand Slams in golf and tennis.

"That all helps the game of tennis."

Federer is still one short of matching Sampras' number of Wimbledon triumphs.

Sampras won seven times between 1993 and 2000 and Laver suggested the American would possibly edge a match against Federer at Wimbledon, if both players were at their peak.

"In some ways I think I might take Sampras, only because of his serve," added the Australian.

"He's got a big serve and volleying ability.

"He's a little more versatile when it comes to the power game.

"It's a hypothetical thing, of being able to say who is going to do it.

"It depends who is in form that day."