Pete Sampras decided to walk away from tennis aged 31 after being burned out from the grind of the Tour.
He does not understand how Roger Federer, 32, does it.
"I was fatigued the last couple years,'' Sampras said on Friday at the Australian Open, where he will present the men's singles trophy on Sunday.
"I feel like my last win (at the US Open) was my last fuel in my tank. That's when I knew I was done.''
Sampras is amazed that Federer is not only playing at the age of 32, but still trying to improve his game, adding Stefan Edberg as coach and switching to a larger racket.
"Seems like he wants to play for another four or five years. The fact that he's able to keep it so fresh is impressive.''
Since retiring, Sampras has not been a fixture at the Grand Slams. He plays a few exhibition events and he has not thought about joining his fellow rivals - Edberg, Boris Becker, Ivan Lendl, Michael Chang - in coaching.
"I miss the game, but I don't miss the stress."