Marat Safin sent top seed Andy Roddick tumbling out of the Australian Open on Tuesday.

The Russian ace celebrated his 24th birthday with a 2-6, 6-3, 7-5, 6-7, 6-4 victory in an explosive quarter-final.

Roddick burst out of the blocks, the U.S. Open champion speeding through the first set in 26 minutes as a nervous-looking Safin committed a string of unforced errors.

But former world number one hit back with a vengeance to take the second and third sets from a bewildered Roddick, who had previously not dropped a set all tournament.

Safin, Melbourne runner-up in 2002, inexplicably suffered a meltdown in the fourth-set tiebreak, allowing the American to run away with it 7-0.

However, the 2000 U.S. Open champion broke Roddick in the ninth game of the final set and, after saving two break points, wrapped up the biggest shock of the men's tournament with a forehand volley after three hours 23 minutes.

Safin will play defending champion Andre Agassi in the semi-finals.

Agassi went through after his opponent, France's ninth seed Sebastien Grosjean, limped out after straining a groin muscle.

"I played a forehand and I felt something really hard in my leg straight away," he said. "It was tough straight afterwards to move. You know, I couldn't move, I couldn't give 100 per cent so I had to stop."

Agassi was leading 6-2, 2-0 when Grosjean informed the chair umpire that he could not continue.

"You know, it's not a good way for anything to end," said fourth seed Agassi after extending his unbeaten run at Melbourne Park to 26 matches.

"I would have preferred to finish the match, no question. You just don't want any match to end that way."

Women's fourth seed Amelie Mauresmo broke down and wept early in the morning when she realised she would not be able to play her match against Colombia's Fabiola Zuluaga because of a back injury.

Zuluaga got a walkover through to the semis -- a first for any Colombian, man or woman -- where she will meet top seed Justine Henin-Hardenne on Thursday.

The Belgian's superior fitness, patience and self-belief lifted her past a sluggish Lindsay Davenport. Having taken an early 4-0 lead, American Davenport wilted under the Melbourne sun, losing 7-5, 6-3.

MAURESMO INCONSOLABLE

France's Mauresmo was inconsolable after being forced to pull out. A finalist here in 1999, she practised early on Tuesday morning under the roof to test a back muscle injury.

All had looked good as she rallied from the baseline with a hitting partner but, when she approached the net to volley, she suddenly winced in pain, holding her side.

As Mauresmo left the court soon afterwards she broke down in tears, burying her head in her towel. She wept again as she was consoled by tournament chief Paul McNamee at courtside.

"It's such a big disappointment for me," the 24-year-old said after informing the organisers she would not play Zuluaga.

Her lips quivering and eyes watering, she added: "I felt I was playing some good tennis and felt I had some opportunities in this tournament."

While benefiting, Zuluaga sympathised with the Frenchwoman.

"It feels good, but I didn't want it to be like this the first time, I would have preferred to play," the 25-year-old said.

Davenport opened her quarter-final with a flourish, racing through the first four games on Rod Laver Arena centre court.

The 2000 Melbourne champion led 5-3 and held three set points at 5-4 in the first set but could not close it out against the French and U.S. Open champion.

"Being up 5-3 and serving for it...to not win the first set is disappointing. I had the opportunities and wasn't able to take them," Davenport said.

LUMBERING DAVENPORT

"You know, I started off playing really great and attacking the balls. We had some close games and I won them, but at 4-0 she hit some great shots and I lost them.

"I started making more unforced errors and she got more confidence. You've got to be able to pull those tight games out and I didn't.

"It's tough. It didn't happen today. I don't have regrets, but I feel like I just maybe went for it a bit too much."

Davenport certainly tried to keep the points short, not wishing to be drawn into long, draining rallies. As the match wore on she became increasingly leaden-footed and had to rely on all-or-nothing winners.

It was a high-risk approach and one which ultimately failed.

"Lindsay was playing unbelievable at the start of the match and I just wasn't ready for that," Henin-Hardenne said.

"But I was able to stay in it. Keep in the rallies and I also did a lot of winners today. I made sure she had no easy shots."