Melbourne:Defending champion Novak Djokovic struggled in the heat on Tuesday before retiring from his Australian Open quarterfinal against American Andy Roddick.
Roddick advanced 6-7 (3), 6-4, 6-2, 2-1, maintaining a sequence of semifinal appearances at Melbourne Park every alternate year since 2003.
Third-seeded Djokovic had just had his service broken to go down 2-1 in the fourth set when he went to the side of the court and told umpire Carlos Ramos he could not continue.
He shook hands with Roddick and left Rod Laver Arena with the trainers.
The outside temperature with the roof open at Rod Laver Arena during the match was as high as 35 Celsius (95 Fahrenheit).
He'd had a medical timeout after recovering a service break to lead 2-1 in the third set and complained of heat-related problems.
Roddick, the 2003 U.S. Open champion, won the next five games to finish off the third set, winning the last 11 points.
Djokovic, who beat Roddick in the quarterfinals at last year's U.S. Open, finally held the opening game of the fourth set after nine minutes but was visibly slowing down and irritated.