Second seed Andy Roddick reached the semi-finals of the Australian Open when opponent Nicolay Davydenko was forced to retire with breathing problems.
The American, who had to save a set point in the second set, was already in control at 6-3 7-5 4-1 when Davydenko indicated he had had enough.
The Russian broke serve twice but could not make it count on either occasion.
Roddick, who will face Lleyton Hewitt or David Nalbandian, has only dropped one set so far, against Greg Rusedski.
Davydenko, who knocked British number one Tim Henman out in the third round, was at a loss to explain what caused his retirement.
The 23-year-old said. "I think at about three all in the first set, I felt something.
"Because we played short points this was okay, but then a few points were long and I felt something. I could not breathe."
Roddick, who coped much better with the heat, said: "Obviously it was pretty toasty out there.
"When he called the trainer, I figured he had to be struggling a little bit, especially when I saw him with the inhaler. It looked pretty serious."
The world number two added: "It's been pretty smooth sailing so far. I don't have many miles on me at this tournament."