China's Li Na battled past sixth-seeded American Venus Williams 2-6, 7-6, 7-5 to advance to the semi-finals of the Australian Open at the Rod Laver Arena in Melbourne on Wednesday.

The victory also ensured her and Zheng Jie became the first Asian players to make the last four of the same Grand Slam.

Li and Zheng's giant-killing run has ignited the draw at Melbourne Park and thrilled fans at home, where tennis, like all sports, was once denounced as bourgeois and banned under the rule of Mao Zedong.

"Right now you can see many media from China right here ... After the match I get 20 text messages from friends because they saw the match," said Li.

Li Na reached her goal of becoming China's first top 10 player quicker than expected after her victory over Venus in three sets.

The 27-year-old, who has battled injuries and Chinese officialdom for years to suddenly achieve her best results on tour, said she hopes the win over the seven-time Grand Slam winner Venus Williams is not her best.

"I don't want to say yes, because the tournament is not finished yet," a beaming Li told reporters after setting up a semi-final with Venus's younger sister Serena.

"So exciting. Maybe I take the beer tonight," she joked.

"Because my goal this year was top 10. But now it's only January so it's come quickly.

Defending champion Serena Williams produced a remarkable fightback from a set and 4-0 down against Victoria Azarenka to reach the semi-finals.

The world number one, initially bullied by the seventh seed from Belarus in their quarter-final, came through 4-6, 7-6, 6-2 and will meet Li Na for a place in the final.

The American admitted she started to doubt herself before she rediscovered her self-belief and found a way to win.

"I'm surprised. I didn't expect to win when I was down in the second set," she said.

"I thought if I lost the singles and the doubles, I could catch a flight on Friday. It's not what a champion is supposed to think but I did."

Azarenka had raced out to a 4-0 lead in the second set before Williams finally rediscovered her power game, footwork and court positioning to win the next five games and the decider.

It was historing repeating itself for the 20-year-old in Melbourne. Last year too Azarenka had led Serena by one set before she was forced to retire with illness

"I didn't change my game, she really went for her shots and she made them. I don't think I did many unforced errors, but it was her controlling some of the points," she said.

World number one Roger Federer reeled off 13 games in a row to dig himself out of trouble and beat Russia's Nikolay Davydenko 2-6, 6-3, 6-0, 7-5 in a wildly fluctuating match to reach the last four.

The world number one was struggling just to stay with Davydenko as the sun started setting and shadows started creeping on to the Rod Laver Arena but seized his opportunity when the Russian suddenly went off the boil.

"I've played him many times and I know he goes through phases, you just try to stay positive," the Swiss said.

"I wasn't playing terribly, just struggling with the sun coming across. It's like a rain delay, you just wait."