Novak Djokovic said there was no way he could have played on against Andy Roddick after quitting their Australian Open quarter-final because of cramp.
Roger Federer said it was "disappointing" that the Serb "gave up" while trailing 6-7 6-4 6-2 2-1 in temperatures hitting 40C on court.
Defending champion Djokovic, 21, has failed to finish seven tour matches, four in Grand Slams.
"I always retire with a reason," he said. "I felt I could not go on."
Djokovic blamed cramp and "soreness in my whole body" as the defence of his Australian Open title ended in miserable fashion.
However, Federer, who later crushed Juan Martin del Potro to book a semi-final against Roddick, said he was not surprised to see Djokovic pull out.
"It's happened before. It's not the guy that's never given up in his career," said the 13-times Grand Slam champion.
"It's disappointing to see when you have two top guys playing each other and you give up.
"He gave up against me in Monaco last year because of a sore throat so those are kind of things you wonder about.
"But I think Andy probably would have run away with the match anyway. Andy completely deserved the win."
Djokovic, who won his first Grand Slam title in Melbourne a year ago, was unhappy with the scheduling of the tournament.
The world number three said he had asked organisers for a night match on Tuesday after finishing his fourth-round match against Marcos Baghdatis at 2.27am local time on Monday.
"I didn't get to sleep until around 5, 5.30am. I didn't really have time to recover. There were really difficult circumstances," he said.
"Conditions were extreme today. It did affect more on me than him, as you could see. But that was the situation. I just have to cope with it.
"It was unfortunate for me. I did request to play night match, but it didn't come up good for me."
I'm almost in favour of saying, you know what, if you're not fit enough, just get out of here
Roger Federer
Traditionally, the Australian Open evening session begins at 7.30pm local time with a women's match followed by a men's singles.
That has led to some late finishes, with Baghdatis and Lleyton Hewitt finishing a match at 4.34am last year.
Djokovic was not the only player to struggle on Tuesday as Melbourne experiences what is expected to be its most severe heatwave in 100 years.
Marion Bartoli later said she had felt the effects of the heat as she lost the final 11 games in her defeat by Vera Zvonareva.
The Australian Open has an extreme heat policy which takes into account the temperature as well as humidity.
In that situation, no new sets are started on outside courts and the roofs on the main show courts are closed.
However, that has not been implemented yet in 2009, despite on-court temperatures topping 40C.
Djokovic was the fourth player to pull out of a singles match in two days, although his was the only retirement caused solely by the heat.
"The main reason is cramping and soreness in the whole body," he said.
"I think the people could see that I was struggling with movement. I couldn't serve the way I served in the first two sets.
"That third set I just started dropping 20, 30 kilometres per hour first serves.
"(It was a) really unfortunate way to end up my Australian Open. I really tried my best, but sometimes you can't fight against your own body."
Roddick said he had not noticed a problem until his opponent called for the trainer at the beginning of the third set.
The American, who has been in superb form in Melbourne despite being written off by many people as a title challenger, said he had no quibble with Djokovic retiring.
"That's sports. That's what makes it fun. There's no script," he said.
But Roddick did call for a change to the medical time-out rule.
The Serb first called for the trainer in the third set when he had just held serve to make it 2-1.
After a 10-minute time-out for Djokovic to receive a massage, Roddick served three double faults as he struggled to hold serve.
"Everything Novak did today was well within his rights and the rules. It's simply about my opinion of a rule," said Roddick.
"If you want to get something on a switchover for cramping, I think that would be OK.
606: DEBATE
These retirements have happened far too many times now. They reflect very poor sportsmanship
random50
"I looked over and I was confused, because I thought it was one injury per timeout, and I saw a calf, a neck, and an arm but I guess cramping is one condition.
"There's obviously... a little bit of grey area there. Hopefully we'll be able to do something about it."
Federer, who serves as the president of the ATP Players Council, agrees with Roddick.
"It's a fine line, isn't it?," said the world number two.
"I really felt when I was coming up the young players abused it, especially against a player like me.
"I'm almost in favour of saying, you know what, if you're not fit enough, just get out of here.
"It's a tough call. I guess we'll speak about it and see what happens."
