Agassi is refusing to predict when the end might come for Federer and Nadal but, with Nadal turning 33 in June, the former world No.1 knows the 17-time major winner's battered body can't play on forever.

"I'm sick of being wrong, quite frankly. These guys are redefining the bar and expectations," Agassi said when asked to predict how much longer the two living legends have left in the tank.

"Watching how Roger goes through his matches, it's like he has another gear you never knew he had and he makes up for other areas that might not be like they once were.

"But he makes a lot of good decisions out there.

"Rafa needs his body a lot more than Roger does just based on how he plays and his urgency, the physicality.

"Roger has more options to put less kilometres on his body.

"So all I can say is that when it comes to an end, it happens fast.

"To say when that's going to happen, it's not possible."

Throw in Federer's 20 majors and Novak Djokovic's 14 and the superstar trio have amassed 51 grand slam singles titles between them, while sharing the top ranking for a staggering 740 weeks over the past 15 years.

"It's an incredible generation. It will be sad when it's gone," Agassi said during a promotional trip to Melbourne for Lavazza.

"But there's always that anticipation of change. Sometimes the disappointment, too, of change, where it kind of stays open for a little while.

"But you can't make those predictions and speculation. You just count your blessings today and see what happens."

AAP