John McEnroe failed to overcome a 17-year age gap as he lost to Marcelo Rios in the quarter-finals of the Masters at the Royal Albert Hall.
McEnroe believes his opponent should not be in the tournament
McEnroe, 47, had suggested Rios was too young to even play in the event before the 30-year-old beat him 6-3 6-2.
The American said afterwards: "He says he can't play on the tour but I didn't see too much evidence of that."
Chilean Rios next faces defending champion Paul Haarhuis, while Goran Ivanisevic will play Cedric Pioline.
Former Wimbledon champion Ivanisevic, who beat Britain's Jeremy Bates 7-6 (7-3) 6-3, said "It's going to be a tough match.
"He is a very talented great player and he's always in the same mood."
"But I'm playing well, although I was a little bit tired and my shoulder was aching a bit because I had a hard match last night."
I would love to come back (to the ATP Tour) but my body refuses to play the whole year
Meanwhile, Dutchman Haarhuis notched up an impressive 6-0 6-4 victory over Spain's Sergi Bruguera.
With three matches in less than 48 hours, McEnroe was feeling the effects of a tough schedule and admitted his playing days could be numbered.
"At this stage the door unlocking its way to further pastures beyond tennis is getting a little bit wider," he added.
"Marcelo's a great player. I wasn't playing bad and had some chances but it was a tough draw to play Marcelo."
Before the match, McEnroe questioned playing Rios, accusing the tour organisers of "bastardising the whole thing" by scrapping the original age limit of 35.
But Rios, two years younger than Britain's Tim Henman who still plays on the main ATP tour, insisted: "I would love to come back but my body refuses to play the whole year.
"That's the reason I'm not playing but I'd still love to come to places like this and play guys like John McEnroe and Goran Ivanisevic."
In the other half of the draw, Pioline beat Henri Leconte in an all-French quarter-final to earn his showdown with Croatian Ivanisevic on Saturday.