Top-ranked Roger Federer survived a sluggish start to knock out Nikolay Davydenko in the quarterfinals of the Australian Open?in Melbourne on Wednesday.

The world No. 1 looked out-of-sorts early on as the in-form Davydenko, who had been unbeaten in 2010, came storming out of the blocks.

However, having lost the first set and facing break points to avoid a second successive break, Federer dug deep to claw his way back into the match.

And having found his form, Federer, who lost to the Russian in Doha, was able to go on to record a 2-6, 6-3, 6-0, 7-5 victory.

"I was in a tough situation at 6-2, 3-1 down and 15-40 on my serve, but that's the beauty of best-of-five sets," Federer said.

"I wasn't panicking, I just relaxed and thought if the sun goes and his level drops just a little bit, the whole thing might change for the better and it did. I couldn't believe the way it changed."

The Swiss star, seeking his 16th Grand Slam title, will play Frenchman Jo-Wilfried Tsonga for a place in the final. Tsonga advanced to the last four after outclassing Serbia's Novak Djokovic 7-6 (8), 6-7 (5), 1-6, 6-3, 6-1.

On the women's side, China's Li Na upset seven-time Grand Slam winner Venus Williams in a three-set thriller to join compatriot Zheng Jie in the semifinals.

The 16th-seeded Li came from one set down to beat the American sixth seed 2-6, 7-6 (4), 7-5, reaching the semi stage of Grand Slam events for the first time in her career.

"This is the best day of my whole life," the 27 year-old Li said.

It is also the first time two Chinese players have made the last eight of the same Grand Slam. The unseeded Zheng eased past Maria Kirilenko of Russia 6-1, 6-3 in her fourth-round clash on Tuesday.

Li will next face defending champion Serena Williams in the semifinal. The women's world No. 1 moved through after staging a huge fightback to down Belarussian Victoria Azarenka 4-6, 7-6, 6-2.