Three matches into the Australian Open and the only thing that has made Serena Williams break into a sweat was the extreme heat of week one at Melbourne Park.

The world No.1 has dropped just 12 games on her way to the last 16 and goes into tomorrow's match with Ana Ivanovic looking every inch a potential six-time Australian Open champion.

From here on, there should be little talk of excessive heat, as forecasters are predicting temperatures in the low to mid 20s, compared to the 40-plus of the past four days which made life uncomfortable for everyone. Petra Kvitova, Sara Errani and Sabine Lisicki all found the heat too much to handle in week one but Williams is still there.

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Admittedly, though, even she struggled just a little yesterday as she beat Daniela Hantuchova 6-3, 6-3. "I feel good to have got through that one because it was tough conditions out there," said the American. "I definitely look forward to playing in the cooler temperatures. I think the players have done really well in the heat. I just look forward to some cooler weather."

Victoria Azarenka and Maria Sharapova were scheduled to play their third-round ties last night but no-one looks better than Williams, who said she was looking forward to stepping up a gear to meet the challenge of Ivanovic. "She [Ivanovic] is doing really well and is on a little winning streak herself [after winning in Auckland last weekend]," added Williams. "I think it will be a great match for me to really start to try to play better in this tournament."

Ivanovic booked her place in the last 16 with a high-quality, dramatic match with Australian Sam Stosur, winning through 6-7, 6-4, 6-2. With rain starting to fall, the Serb netted a forehand at 7-7 in the tiebreak and, after failing to have the point replayed, waited until the roof was closed and promptly lost it 10-8.

Yet the former world No.1 picked herself up well, thumping her forehand to great effect to see off Stosur, a former US Open champion whose hopes of success on home soil were dashed once more. "I'm really happy with the way things are going," Ivanovic said. "I worked so hard and I know what I'm doing on the court. So that definitely helps me relax. Just embrace the moment."

Williams has won all four of their previous meetings in straight sets and Ivanovic acknowledged that she would be in for a serious test. "Serena is on top of the game for so long now," she said. "She's someone we admire, actually. It's going to be a very tough task. But I look forward to that challenge. I've been playing some good tennis all summer. It's going to be nice test to see where I'm at."

The third seed Li Na, a probable semi-final opponent for Williams or Ivanovic, flirted with disaster before scraping past the Czech Lucie Safarova 1-6, 7-6, 6-3. The Chinese, who reached the final here in 2011, was outplayed at first but battled hard, saving a match point at 6-5 in the second set and then recovering for victory.

On the match point, Safarova's backhand down the line landed only centimetres too long and the former French Open champion admitted she had ridden her luck. "I think the five centimetres saved my tournament," said Li, who now plays Russia's Ekaterina Makarova.

"If she hit it in, my whole team would have been on the way to the airport. I was really happy the way I fought on the court from the first point until the last point."

Australia do still have one woman left in the draw after the left-hander Casey Dellacqua eased past a heat-affected Zheng Jie 6-2, 6-4, setting up a match with the Canadian teenager Eugenie Bouchard. The ninth seed Angelique Kerber will take on Italy's Flavia Pennetta in the remaining last-16 match.

Everything appears to be going smoothly for Novak Djokovic in his quest to become the first man in the Open era to win the title four years in a row. The Serb waltzed past Denis Istomin of Uzbekistan, 6-3, 6-3, 7-5, his only moment of concern coming when he failed to serve out for victory at 5-4 in the third set.

Seeded second behind Rafael Nadal - he was scheduled to play France's Gael Monfils this morning, UK time - it was only a blip as Djokovic duly broke serve in the next game and served out to clinch victory. "I'm my biggest critic but I have to be satisfied with each performance," Djokovic said. "As the tournament progresses I'm playing better and better."

His opponent in the last 16 will be Italy's Fabio Fognini, while Spain's Tommy Robredo outgunned the ninth seed Richard Gasquet 2-6, 7-5, 6-4, 7-6 to set up a meeting with No.8 seed Stanislas Wawrinka. He received a walkover after an injury to Vasek Pospisil of Canada.

Tomas Berdych, sporting a shirt which looked more like a Sheffield Wednesday football jersey, cruised past Bosnia's Damir Dzumhur and now plays Kevin Anderson, while Florian Mayer took out a hapless Jerzy Janowicz to set up a meeting against third seed David Ferrer.

There was a defeat for the former Wimbledon doubles champion, Jonny Marray, though, as he and Australia's Paul Hanley were beaten 6-4, 7-6 by the world No.1 pair, Bob and Mike Bryan in round one.