* Djokovic, Serena charge into fourth round
* Temperatures plummet in Melbourne
* Li survives scare, Robredo upsets Gasquet
MELBOURNE, Jan 17 (Reuters) - Novak Djokovic bared his teeth to finish off Denis Istomin and extend the hottest winning streak in tennis to 27 matches in the third round of the Australian Open on Friday as a cool front brought respite to Melbourne after a four-day heatwave.
Serena Williams earlier made light of the last of the scorching temperatures, which again peaked around 42 degrees Celsius (110 degrees Fahrenheit), to blast her way into the fourth round for the loss of only a dozen games.
China's Li Na escaped defeat by a matter of centimetres in her third round match, while Spain's David Ferrer continued carving his way through the men's draw largely unnoticed.
Ferrer's compatriot Tommy Robredo caused the biggest upset of the day by edging ninth seed Richard Gasquet 2-6 7-5 6-4 7-6(6) in an evening thriller disrupted briefly by rain.
Djokovic's last defeat was against Rafa Nadal in the U.S Open final and 49 Istomin never looked like stopping his winning run at 26 matches under the closed roof of the Rod Laver Arena.
Looking to become the first man in the professional era to win four consecutive Australian Opens, second seed Djokovic raced through the first two sets in 70 minutes before the Uzbek suddenly upped his aggression to break the Serbian as he was serving for the match.
Visibly piqued, Djokovic was in no mood to risk the possibility of giving up his first set of the tournament and broke back immediately before overwhelming Istomin with the power of his shots to convert his second match point.
"I feel better on the court as the tournament is progressing," said Djokovic, who will take on his Italian friend Fabio Fognini for a place in the quarter-finals.
"Things got tense towards the end of the match. I didn't want to drop the third set, obviously, so I was very focused to get the job done in straight sets."
Temperatures are forecast to plummet on Saturday, a huge relief to players and organisers alike after four days of rows over what constitutes dangerous conditions.
"I definitely look forward to playing in the cooler temperatures," said Williams, whose 6-3 6-3 win over Daniela Hantuchova was a record 61st at the Australian Open, taking her past local great Margaret Court's 60.
That Williams achieved her victory without firing on all cylinders - "I wasn't fully on today" - was another warning to her rivals that her 11-title 2013 season was by no means a final flourish at the end of her career.
"For whatever reason, I feel like I just never was really able to reach my full potential, and I feel like recently I just have been able to do a little better," she added.
The next hurdle in the 32-year-old's path to a sixth Australian Open title is Ana Ivanovic after she ended Sam Stosur's hopes of winning her home grand slam for another year with a 6-7(8) 6-4 6-2 victory on Rod Laver Arena.
"I've been playing some good tennis all summer. It's going to be nice test to see where I'm at," Ivanovic, who squandered a 6-3 lead in the first set tiebreak against Stosur, said of her meeting with the 17-times grand slam champion.
Ferrer's attritional style was made for hot conditions and he survived a second set wobble to wear down Frenchman Jeremy Chardy and reach the fourth round at a 15th consecutive grand slam with a 6-2 7-6 (7-5) 6-2 victory.
Ranked and seeded third after Andy Murray missed the back end of last season, Ferrer next faces Florian Mayer with Tomas Berdych or Kevin Anderson waiting in the quarter-finals.
Anderson came from two sets down for the second time in five days to reach the last 16, outlasting Edouard Roger-Vasselin 3-6 4-6 6-3 7-6 (7-5) 7-5 in just under four hours.
The tall South African trailed 0-4 in the fourth set tiebreak and 3-5 in the fifth and let out a huge roar when the Frenchman went long to hand him the win on Margaret Court Arena.
"It was a lot of emotions at the end," said Anderson. "Just a lot of excitement. A little bit of disbelief as well.
"Sitting down there at 4-5 9in the (fifth), going out to return serve, knowing sort of it's your one last shot."
Berdych, by contrast, eased into the fourth round without having dropped a set all week courtesy of a 6-4 6-2 6-2 victory over Bosnian qualifier Damir Dzumhur.
With Nadal, Roger Federer and Murray in the other half of the draw, Berdych knows he will rarely have a better chance to reach a first semi-final at Melbourne Park.
"You know, it's always about the draw," said the Czech seventh seed. "Really, if I'm going to have it once pretty good, yes, I take it."
Li staged the most dramatic escape of the day when she faced a match point against Czech Lucie Safarova, who failed to convert it by the smallest of margins when her backhand down the line was called out, a decision confirmed by Hawk-Eye.
"I think five centimetres saved my tournament," said Li, who rallied to win the subsequent tiebreak and then seal a 1-6 7-6 (7-2) 6-3 victory. "If she had hit it in, the whole team would be on the way to the airport." (Editing by John O'Brien and Martyn Herman)