Andre Agassi believes Roger Federer's sublime form has proven the Swiss Master can land a fifth Australian Open crown and continue adding to his grand slam title haul for years to come.
More than a decade after losing the 2005 US Open final to Federer at 35, Agassi can't believe the 17-times major winner is still delivering vintage performances at the same age.
Turning 36 in August, Federer crushed world No.10 Tomas Berdych 6-2 6-4 6-4 on Friday night in only his third official match back after six months out of the game with a knee injury.
"Roger has gotten me to stop predicting anything, quite frankly," Agassi told AAP ahead of Federer's fourth-round Open clash with fifth seed Kei Nishikori on Sunday.
"Five, seven years ago, I never in a million years would have thought that he could sort of look and be at the level you can see that he's at.
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"With that being said, he has made the game look rather easy. But you've got to work hard to make it look easy.
"He has so many opportunities to fall back on, ways to play, to shorten points. "He can just get out there and neutralise a Berdych, as an example, and move him forwards and backwards and playing so many different varieties of shot."
Like Federer, Agassi reigned four times in Melbourne, before retiring after the 2006 US Open at 36.
The American said fans should enjoy watching Federer in full flight while they can.
"The end happens fast," Agassi said after promoting his role as Lavazza Global Ambassador.
"So it is remarkable (what Federer is doing) and he might do it for a number more years and he can win more (slams), not just one.
"And if things fall away, then in my experience, when it turns, it turns."
Victory over Nishikori could thrust Federer into a quarter-final blockbuster on Tuesday with world No.1 Andy Murray.
