Andy Murray reached the semi-finals of the Australian Open for the first time on Tuesday when defending champion Rafa Nadal quit with a right leg injury midway through the third set.

The Scotsman produced a breathtaking display to open up a 6-3, 7-6, 3-0 lead when Nadal, who had called in the trainer to treat the area just above his knee earlier in the set, shook his head and informed Murray that he could no longer continue.

Nadal's exit was a sad note to a match of the highest quality and will raise doubts about his ability to return to the incredible standards he reached in winning six grand slam titles.

He is still only 23 but has been plagued by knee problems over the past nine months after being on top of the world a year ago when he held the Australian Open, French Open and Wimbledon titles and was ranked number one.

Nadal's forehand used to be the most destructive weapon in the game and he would dictate play by pinning his opponents behind the baseline.

The Spaniard has slimmed down after being plagued by injuries over the past year. He still gets around the court quickly but does not overpower opponents like he used to.

But his retirement on Tuesday should not detract from the performance of Murray, who is in the best form of his life, and a match that whipped the centre court crowd into a frenzy.

Murray's win would have given him real hope that he could be on the verge of ending Britain's 74-year wait for a men's grand slam champion and he will take on Croatia's Marin Cilic for a place in the final.

"I've known Rafa since I was 13 or 14 and he is somebody I have always looked up to," Murray said in a courtside interview.

"He is my favourite player to watch because of his energy and I am gutted for him.

"I came through a few difficult moments at the start of the match but I played well and I got my tactics right."

Murray relies less on brute force, combining his strength with some delightful soft touches and is a master of the drop shot.

He has always been able to fetch more balls than a golden retriever but has worked hard on his fitness over the past two years to last the distance against top players.

The fairytale continues for Justine Henin.

Through to the semi-finals of the Australian Open, the Belgian's dream of winning the championship is edging closer to reality.

Henin booked her place in the last four with a 7-6, 7-5 win over Russia's Nadia Petrova on the Rod Laver Arena.

As the scoreline suggests, it was a tight match but Henin remains an expert at finding a way to win the close ones.

The former world number one is playing only her second tournament since coming out of retirement and her victory was another triumph for her golden generation.

Henin was partly inspired to return to the tour while watching Clijsters win last year's US Open shortly after her return. Henin needed a wildcard for Melbourne but never gave any serious thought to winning the championship, until now.

"I was curious about what was going to happen," she said. "I knew it was going to be hard. Every match was a goal.

"I never thought, I'll be in the semis.

"Now I can dream of being in the final of this grand slam, of course, but it's still a long way.

"I just have to try to be focused on what I have to do on the court and hope it's gonna keep going this way."

Croatia's Marin Cilic is rapidly becoming the marathon man at the Australian Open after his third five-set win of the tournament propelled him into his first grand slam semi-final.

The 21-year-old beat an injured seventh seed Andy Roddick 7-6, 6-3, 3-6, 2-6, 6-3 on Tuesday, and extended his stay on the blue Melbourne Park courts at least for another round.

Though given his virtual permanent residency at the venue with the match against Roddick taking three hours, 50 minutes, and extending his time on court at the tournament to a touch over 18 hours, he may consider applying for an Australian passport.

Cilic had needed a fifth set to beat Australian teenager Bernard Tomic in the second round and was then taken to another decider by US Open winner Juan Martin Del Potro in the fourth.

The 14th seed, who defended his Chennai title before the Australian Open, also extended his winning streak to 10 matches and now meets either defending champion Rafa Nadal or fifth seed Andy Murray for a place in the final.

"Today was not easy," Cilic told reporters. "I was also a little bit tired from the matches before.

"(The) first set (took) a lot of energy out of me because there was not only (the) physical battle but also a mental battle, the first set, to gain the advantage.

"So that was most important to win for me." 

Roddick had suffered pain in his right shoulder, that extended down his arm and into his hand, though it did not hamper the American who managed to send the match into a decider, which caused Cilic some consternation.

"He started to play pretty good in the beginning of the third," said Cilic.

"(He) was putting a lot of pressure on me. Then I got lost a bit. It wasn't easy to come back.

"So the fifth set I think ... turned around for me when I saved those three break points in the first game (of the set).

"It was a little bit of a relief. I just was thinking to play point by point, to try to stick with him, and I hopefully get my chance, which I did."

Roddick praised Cilic's unflappable nature after the match, "he keeps it together real well", which the Croat said was part of his make-up though he was now entering unknown territory.

China's Zheng Jie advanced to her second grand slam semi-final after she demolished Russia's Maria Kirilenko 6-1, 6-3 at the Australian Open on Tuesday.

Zheng, who became the first woman to advance to a grand slam semi-final as a wildcard at Wimbledon in 2008, will now meet former world number one and seven-time grand slam winner Justine Henin for a place in the final.

Kirilienko, who had her left thigh heavily strapped before the match started, was stretching and rotating her right hip during the first set and had to take an injury time out between sets and also received treatment several times during the second.

"I feel a bit calmer than the last time I made a semi-final," said Zheng.

"At Wimbledon, I didn't have enough confidence. I didn't have that 'must-win' attitude... I didn't think it was possible for me to make the last four and meet the best players.

"Now, it's different, it's not like last time where I was 'ugh, I don't know what to do, but I'll just do it'.

"Now, I feel like whoever makes the last four has a chance of winning it