Australian Open [Images] champion Novak Djokovic [Images] overcame the distraction of two violent thunderstorms to beat Croatia's Mario Ancic 6-3 6-4 on Thursday in the quarter-finals of the Sydney International.

The Serbian world number three kept his cool as the skies opened up above Sydney's Olympic [Images] tennis centre with two heavy downpours after a baking hot day where the temperature soared to 41.2 degrees Celcius.

Djokovic won five games in a row after trailing 3-1 to wrap up the first set, then broke Ancic's serve early in the second before the foul weather intervened.

Play was halted for almost two hours and organisers almost postponed the match until Friday morning before the showers cleared long enough for the players to finish.

"I was thinking it would be better for both of us, whoever wins the match, to end up tonight so he can rest for the semi-final," Djokovic said.

"I think as a winner I can say it's much better to finish tonight."

Djokovic, who needed a wildcard entry into Sydney after his Australian Open preparations came unstuck with an unexpected first-round loss in Brisbane last week, plays Jarkko Nieminen in Friday's semi-finals.

The Finn got a free-ride into the last four when his opponent, Frenchman Jo-Wilfried Tsonga, pulled out before their match with a back injury.

"I decided this afternoon because the pain, it's too much, I cannot play," Tsonga told a news conference.

"I saw my physio a lot yesterday. I saw the physio from the ATP. I saw the doctor and they told me it's difficult to play like that."

Tsonga, runner-up to Djokovic at last year's Australian Open, has been troubled by back problems in the past and was unsure whether he would be able to take his place in the first grand slam of the year, starting in Melbourne on Monday.

"I hope it will not affect me for the Australian Open but I don't know," he said.

"I can't do anything for the next two days. I will stay maybe in my bed and hope I will be better."

Argentine David Nalbandian [Images] beat four-times Sydney champion Lleyton Hewitt [Images] 7-6 7-5 to book his place in the semi-finals against Richard Gasquet, a 6-2 7-6 winner over fellow Frenchman Jeremy Chardy.

Those two matches were played in the afternoon session when the weather had cooled and the wind was starting to pick up as the storm closed in.

"It was a very strange day. We warmed up with 40 (degrees) and it was so hot we couldn't breathe," Nalbandian said.

"Then one minute it was completely different, cold, so windy, cloudy. It was like two tournaments."

Hewitt, who has just returned to the circuit from hip surgery, said he was encouraged by his performance against the number 11 ranked player in the world.

"I'm as ready as I think I thought I'd ever be coming into this situation," he said.

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