MELBOURNE - Defending champion Novak Djokovic charged into the Australian Open fourth round Friday and Serena Williams smashed the 39-year-old record for wins at the tournament as a sweltering heatwave finally eased. Djokovic, gunning for a fourth straight victory at the season's first Grand Slam, simply had too much firepower against Uzbekistan's Denis Istomin as he racked up his 24th consecutive win in Melbourne.The world number two is also unbeaten since last year's US Open, making him narrow favourite over hot rival and 13-time Grand Slam champion Rafael Nadal, whom he cannot meet until the final. Djokovic, 26, eased through the first two sets and, after being broken for the first time when serving for the match, he broke straight back and wound it up 6-3, 6-3, 7-5. "Things got tense towards the end of the match. I made some double-faults. This is what happens," he said. "I could have easily got into a tiebreak and then it's very even, very unpredictable who is going to win the third. I didn't want to drop the third set, obviously, so I was very focused to get the job done in straight sets."Djokovic, playing in the evening, benefited in a sharp drop in temperatures which have topped 40 Celsius (104 Fahrenheit) for four days straight, one of Melbourne's most severe heatwaves. Earlier, Williams opened the day's events on a baking hot Rod Laver Arena but she appeared untroubled as she beat Daniela Hantuchova 6-3, 6-3 to reach the fourth round for the loss of 12 games. Shielded from the sun by a pink cap, the 32-year-old world number one wielded a killer serve as she overpowered the Slovak in 80 minutes, setting up a meeting with Serbia's Ana Ivanovic. It was Williams' 61st victory at the tournament, breaking the record set by 11-time champion Margaret Court up to 1975. Williams' first Australian Open match win was on her debut aged 16 in 1998. Asked which had been her favourite victories, she deadpanned: "All the finals I was able to win." China's Li Na soon joined Williams in the next round but she took a very different route after flirting with disaster in a three-set struggle with Lucie Safarova. Safarova narrowly missed a forehand on match point in the second set and Li then played a nerveless tiebreak before taking it 1-6, 7-6 (7/2), 6-3 in two-and-a-half hours. "I think the five centimetres (two inches) saved my tournament," said Li, referring to Safarova's miss on match point. "If she hit it in, I think my whole team are on the way to the airport.Li will now play Russia's 22nd seed Ekaterina Makarova, who beat Romania's Monica Niculescu, while Italian Flavia Pennetta beat Mona Barthel to set up a clash with Germany's Angelique Kerber. In the evening session, Australia's Sam Stosur suffered further heartbreak at her home Grand Slam when she threw away a one-set lead against Ivanovic. Stosur, who held her nerve when a first-set tiebreak was interrupted by rain -- while she was on set point -- then faded as Ivanovic won 6-7 (8/10), 6-4, 6-2 to set up a meeting with Williams.In 12 visits to the Australian Open, 2011 US Open champion Stosur has never managed to get past the fourth round. In the men's draw, Spain's super-fit Ferrer again proved impregnable as he ground down France's Jeremy Chardy 6-2, 7-6 (7/5), 6-2 to reach his 15th consecutive Grand Slam fourth round. Seventh seed Berdych ended the fairytale run of Damir Dzumhur, the first man from Bosnia and Herzegovina to compete at a Grand Slam whose brave performances won fans at home and abroad.Berdych will next play South Africa's Kevin Anderson, who came through a five-set scrap with Edouard Roger-Vasselin of France. Italian seed Fabio Fognini beat American Sam Querrey in straight sets to set up a last-16 clash with Djokovic. Despite day-time temperatures of 42 Celsius, organisers opted not to repeat Thursday's use of emergency rules which allow them to suspend play on outside courts and close the retractable roofs on the two main stadiums. China's Zheng Jie was the latest casualty when she needed medical treatment and a rub-down with ice during her loss to Australia's Casey Dellacqua. However, temperatures dropped nearly 20 Celsius by the evening and they were forecast to stay low over the weekend.