Like most, Agassi has marvelled at Federer's vintage run through the Australian Open draw in his first official tournament back after an injury-enforced six-month layoff.
But, speaking from experience, Agassi also knows Father Time waits for no-one.
Agassi was 35 -- the same as Federer now -- when the two clashed in a memorable 2005 U.S Open final in New York.
"To speak sobering about it all, when the end comes, it happens fast. It's not subtle," Agassi told AAP from Las Vegas.
The eight-time Grand Slam champion recalled how his loss to Federer -- after he'd survived three successive energy-sapping five-setters to reach the final -- proved the beginning of the end for him.
"Fast forward a month and a half and I sprained an ankle and I can't train the way I need and I'm not ready to be down in Australia because my legs aren't ready," he said.
"I sit that one out and now I'm feeling the pressure of having to perform on the hard courts heading into the clay.
"I know that clay's not realistic for me and so the pressure gets to me over the hard courts. I don't put up good results and now I have one last stretch of hard courts (before the US Open).
"The end happens fast."
Still believing Federer can add to his all-time record 17 Grand Slam singles titles, Agassi isn't expecting the Swiss superstar to suffer the same abrupt decline as he did. But he hopes fans lock in and savour his rare gifts, just in case he does.
"Roger has gotten me to stop predicting anything, quite frankly," Agassi said.
"Because 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 (at 35).
"So it is remarkable and he might do it for a number more years and he can win more Slams, not just one.
"(But) if things fall away, then my experience, when it turns, it turns."
Agassi says fans may only truly appreciate the golden era of tennis they've been treated to during the dominant, decade-long reign of Federer, Rafael Nadal and Novak Djokovic when the trio of titans are gone.
"I don't envy the generation that's trying to take the game forward because the standard of this generation has been remarkable," he said.
Between them, Federer, Nadal and Djokovic have won an incredible 43 major singles titles in the past 11 years.