Serena Williams saved three match points and Novak Djokovic revived his stuttering Australian Open preparations in fierce heat at the Sydney International.

Williams, who also saved four match points in her first round win over Australia's Samantha Stosur, provided a further demonstration of her fighting spirit in coming back to beat Denmark's Caroline Wozniacki 6-7 6-3 7-6 in their quarter-final.

Wozniacki looked poised for a shock win when she broke Williams to lead 6-5 in the final set but she failed to serve out, squandering three match points in the process.

'I was a little angry that I was even in that situation today, because I felt like I probably shouldn't have been in that situation,' Williams told reporters.

'But I was composed. I just felt like I didn't really have anything to lose too much.'

Djokovic rebounded from his shock first-round loss in Brisbane last week to crush Frenchman Paul-Henri Mathieu 6-1 6-2 in their second-round match at the Olympic tennis centre.

He played during the hottest part of the day and although he was not completely satisfied with his performance, he said the workout was just what he needed heading into Melbourne next week where he will start the defence of his title.

'It's absolutely normal that you have some ups and downs during the match and during the tournament, but if you're able to overcome that then you are a champion,' Djokovic said.

Djokovic faces Mario Ancic in Thursday's quarter-finals after the Croat beat Spain's Tommy Robredo, with the winner to play either Jo-Wilfried Tsonga or Jarkko Nieminen of Finland.

Richard Gasquet and qualifier Jeremy Chardy set up an all French quarter-final while Lleyton Hewitt advanced to a potentially explosive clash with his Argentine rival David Nalbandian.

'(Our rivalry) is not something I'll be focusing on going into the match tomorrow,' Hewitt said.

'For me, it's more a good step to see where I'm at going into Melbourne. Obviously the big picture is next week.'

Olympic champion Elena Dementieva chalked up her eighth straight win this season, 6-2 5-7 6-4 over Poland's Agniezska Radwanska, to earn a semi-final with Williams.

World number three Dinara Safina defeated Alize Cornet 6-3 6-4 to set up a meeting with Sugiyama.