Novak Djokovic's reign as Australian Open champion ended in agony today as the "extreme conditions" forced him to quit his quarter-final against Andy Roddick in the fourth set.

The third seed was trailing 6-7, 6-4, 6-2, 2-1 when he threw in the towel after two hours and 29 minutes on a day when temperatures hit 95°F.

Djokovic had a medical timeout after recovering a service break to lead 2-1 in the third set and complained of heat-related problems. The Serb draped towels packed with ice around his neck during changeovers, and lingered in the shade behind the baselines as long as possible.

"It was really difficult, conditions were extreme," said Djokovic.

His previous match against Marcos Baghdatis did not finish until almost 2.30am on Monday morning, having started late because of Jelena Dokic's three-set marathon against Alisa Kleybanova. That late finish, coupled with the searing heat, took its toll on the player who beat Jo-Wilfried Tsonga in last year's final.

This not the first time the 21-year-old has quit in a Grand Slam. He failed to finish his quarter-final against Nadal at the 2006 French Open due to a back problem and his semi-final against the Spaniard at Wimbledon in 2007 because of a foot injury.

Although Roddick coped better today the bad news for him and the remaining players is that it is set to get even hotter with Melbourne prepared for a three-day heat-wave when daily temperatures are expected to exceed 105°F.

Roddick had sympathy for his opponent and said: "To be honest I didn't know (he was sick) until he called for help there. That's disappointing. I feel bad for Novak right now.

"He worked so hard for this last year. To not get a fair chance to defend his title, that's too bad."

The No7 seed from America will now play either Roger Federer or Juan Martin del Potro, who were meeting in today's late match.

Djokovic is the fourth player in two days to retire from a big singles match at Melbourne Park. In the fourth round yesterday Serena Williams advanced when 19-year-old Victoria Azarenka, dizzy and in tears, pulled out in the second set because of a virus.

Williams next plays No8 seed Svetlana Kuznetsova, who went through when Wimbledon semi-finalist Zheng Jie injured her left wrist in a fall and retired two games later at 2-1.

Gilles Simon made the semi-finals for the first time at a major when his friend and fellow Frenchman Gael Monfils withdrew because of an injured wrist after the third set.

One match that was completed today saw Vera Zvonareva run off 11 straight games as she cruised through to the semi-finals with a 6-3, 6-0 win over 2007 Wimbledon finalist Marion Bartoli.

This is Zvonareva's best performance in a Grand Slam and the 24-year-old has reached the last four without dropping a set.

"I'm very excited," said the seventh-seed, who cut her unforced errors from 15 in the first set to two in the second. "It was a great day for me."

Bartoli, seeded 16th, had knocked out No1 seed Jelena Jankovic in the previous round.

"I think she played just unbelievably well," said the Frenchwoman of the last 11 games. "She barely missed one ball after that. I was hitting as hard as I could. She was always coming back with some better shots."

Jamie Murray's Melbourne campaign ended today 24 hours after his brother Andy was knocked out of the singles by 14th seed Fernando Verdasco.

Jamie and Liezel Huber of America, who were the fourth seeds, lost on a champions tie-break 6-7, 6-2, 10-4 to France's Nathalie Dechy and Israel's Andy Ram in the second round of the mixed doubles.