Federer breaks his losing streak against Russian Davydenko [GALLO/GETTY]
World number one Roger Federer withstood a stern challenge from Nikolay Davydenko before mounting a furious counter-attack to win 2-6, 6-3, 6-0, 7-5 and advance to the Australian Open semi-finals for a 23rd straight Grand Slam.

Down a set and a break in the second, the Swiss top seed imposed himself winning 13 consecutive games to take the next two sets, then keeping the Russian at bay in the fourth as shadows crept across Rod Laver Arena.

Federer snapped Davydenko's 12 match winning streak, which included victories over Federer in their previous two meetings.

Serena Williams advanced to the semi-final in Melbourne, despite her older sister Venus' upset loss to China's Li Na in the previous match on Rod Laver Arena.

Four-times champion Serena was down a set and 4-0 in the second before she rebounded for a 4-6, 7-6 (4), 6-2 win over Victoria Azarenka of Belarus.

Fed express

"I played him many times and I know that he goes through phases at times, very rock solid phases," Federer said of Davydenko in a courtside interview.

"I knew he wasn't going to play through three or four sets like that. I knew he was going to dip a little bit."

"Somewhere you hang onto it. (It was) a bit of a surprise to come back that great."

Federer will meet Jo-Wilfried Tsonga in his semi-final. Tsonga fought back from two sets to one down to reach his first grand slam semi-final in two years by beating third seed Novak Djokovic 7-6, 6-7, 1-6, 6-3, 6-1.

Tsonga, who was defeated by Djokovic in the final here in 2008 and had not advanced past the quarter-finals of a grand slam since, looked down and out after the third set but rallied to beat the Serb after three hours 52 minutes.

It was the second successive five-set victory for the 10th-seeded Frenchman, who until his fourth-round win over Nicolas Almagro on Monday had never played a five-set match before.

Venus Williams couldn't make it to an expected sister semi-final, losing in a mistake-filled match featuring in a combined 110 unforced errors against Li.

The result is that Serena will take on Li, who beat Venus 2-6, 7-6 (4), 7-5.

Justine Henin, playing in her first Grand Slam tournament in two years after coming back from a two-year retirement, will play Zheng Jie in the other semi-final.

Serena Williams was far from confident after the first hour of the match, thinking of her sister's loss.

"It was obviously on my mind,'' Serena said of Venus' defeat.

"I saw maybe one or two points, maybe three. I don't want to watch too much, I get too nervous watching. Obviously I was incredibly disappointed.''

Tentative

Serena started off tentatively.

"I wasn't playing my best, especially in the first two sets,'' she said.

"I was down the whole match from the first point. I wasn't surprised but I was definitely shocked.''

Instead of wilting, she fought back from her big deficit in the second set with two service breaks of her own, easily won the tiebreaker and dominated the third.

"I think it's impressive the way she does it,'' Azarenka said.

"She's a strong girl. She has very powerful shots.''

Serena celebrates her comeback win against Azarenka [GALLO/GETTY]
The 20-year-old Belarussian was undone by a trademark comeback by Williams, who is the only player to save match points on the way to three Grand Slam titles. She did it at Wimbledon last year and at the 2003 and 2005 Australian Opens.

Initially, Azarenka looked like she'd continue the Williams family woes.

She broke Serena in the opening game and then, after fending off three break
points, held her own opening serve in a game that lasted more than 15 minutes and went to deuce nine times.

It was mostly one-way until Williams started her comeback in the second - winning five straight games and then getting on top again at the end of the tiebreaker. She dominated the third set, converting both her break-point chances.

Li and Zheng were the first Chinese pair to reach the quarterfinals at the same Grand Slam. Add semi-finals now, and could the final be possible?

Serena Williams and Henin, with a combined 18 Grand Slam singles title between them (Serena 11, Henin 7) would seem to have the inside edge. But Li, who lost the first set and was twice down breaks in the second, says anything can happen.

"In China, we say if you have tough time and then you return back, maybe have good luck,'' Li said.

Shell-shocked

A shell-shocked Venus Williams struggled to make sense of her stunning quarter-final loss to China's Li after blowing a one-set lead and her service game to bow out of the tournament.

Williams, who has not progressed past the last eight at Melbourne Park since 2003, crumbled when serving for the match at 5-4 in the second set in a 2-6 7-6 7-5 upset.

The sixth seed also failed to capitalise on five service breaks in the decider at Rod Laver Arena.

"It's important to put the ball in the court,'' said Venus Williams, who served for the match in the second set.

"I felt like sometimes I made some errors.''

China's Li brings down an error-strewn Venus to advance to the semi [GALLO/GETTY]
That was an understatement. The pair's unforced error count went over the
100 mark midway through the final set, which produced eight service breaks in the first 10 games.

Li, who is forecast to go into the WTA's top 10 rankings after the tournament ends, was dominant in the third set when it counted against Venus Williams, who has five Wimbledon titles and two at the US Open.

"In tennis you have to close it out. It's not like there's a clock ticking and then suddenly it's over. You just have to close it out. I didn't do that today," said Venus.

Despite making the last eight, Venus's progress in Melbourne had been far from smooth.

Wins over Austria's Sybille Bammer, local hope Casey Dellacqua and Italy's Francesca Schiavone were plagued by poor serving and lapses in concentration.

Against Li, Williams managed to land just 56 percent of her first serves and struggled frequently with her ball toss.

"I think I just started rushing my shots a little too much instead of just taking my time and really just hitting a clean ball," said the American.

"Sometimes I was able to back off. But, you know, sometimes it's so easy to get so eager."

Source: Agencies