Novak Djokovic of Serbia makes a forehand return to Denis Istomin of Uzbekistan during their third round match at the Australian Open tennis championship in Melbourne, Australia, Friday, Jan. 17, 2014. (AP Photo/Andrew Brownbill) Li Na of China, right, shakes hands with Lucie Safarova of the Czech Republic at the net after Li won their third round match against Li Na of China at the Australian Open tennis championship in Melbourne, Australia, Friday, Jan. 17, 2014. (AP Photo/Rick Rycroft) David Ferrer of Spain celebrates after winning over Jeremy Chardy of France during their third round match at the Australian Open tennis championship in Melbourne, Australia, Friday, Jan. 17, 2014. (AP Photo/Aaron Favila) Ana Ivanovic of Serbia celebrates a match point won over Samantha Stosur of Australia during their third round match at the Australian Open tennis championship in Melbourne, Australia, Friday, Jan. 17, 2014. (AP Photo/Aaron Favila) MELBOURNE, Australia (AP) - Novak Djokovic extended his winning streak to 27 matches and stayed on track for a fourth consecutive Australian Open title with a 6-3, 6-3, 7-5 win Friday over Denis Istomin. Djokovic, whose last defeat came against Rafael Nadal in the U.S. Open final in September, is aiming to be the first player to win four Australian Open titles in a row in the Open Era. And he is looking the part, despite being broken in his first attempt to clinch the match against Istomin. He has won all three matches so far in straight sets and his major threats - top-seeded Nadal, Wimbledon champion Andy Murray and 17-time Grand Slam singles champion Roger Federer - are in the other half of the draw. "I have to be satisfied ... as the tournament progresses I'm playing better and better," Djokovic said. It's important not to underestimate any opponent. Having the right approach to the court, having the right intensity, starting from the first point to try to show to your opponent that you're there to win. Before rain cooled things off at Melbourne Park, Serena Williams persevered through a fourth day of searing temperatures to also advance. Five-time champion Williams equaled Margaret Court's Australian Open mark of 60 match wins with her second-round victory, and beat that in the third round when she also matched Lindsay Davenport's record of 69 main-draw matches here in the Open era. On Sunday, she'll play former No. 1-ranked Ana Ivanovic, who beat 2011 U.S. Open champion Sam Stosur 6-7 (8), 6-4, 6-2. Williams withdrew from the doubles with her sister Venus, avoiding any extra exposure to the blazing sun during the once-in-a-century heat wave that had gripped Melbourne for nearly all of the first week. The temperature hit 39 Celsius (102 Fahrenheit) during Williams' match and topped 42 C (108 F) by the mid-afternoon, but humidity remained low and there was no reason for organizers to invoke the Extreme Heat Policy and suspend matches for the second day in a row. It dipped to 32 C (90 F) before the night matches as a cool change kicked in, and continued to drop before play ended Friday. The forecast for the weekend and most of next week was for cool and cloudy conditions. In earlier women's play, Li Na, a two-time finalist here and a major threat to top-ranked Williams in the top half of the women's draw, had to save a match point before coming back to beat No. 30 Eugenie Bouchard beat American Lauren Davis 6-2, 6-2 to set up a fourth-round match against Australian wild-card entry Casey Dellacqua, who beat 2010 semifinalist Zheng Jie of China 6-2, 6-4. In night men's matches, No. 15 Fabio Fognini beat American Sam Querrey 7-5, 6-4, 6-4 and will play Djokovic on Sunday in the fourth round. 17 Tommy Robredo defeated No. 9 Richard Gasquet 2-6, 7-5, 6-4, 7-6 (6) and to set up a fourth-round match against a well rested Stanislas Warwinka. Wawrinka received a walkover Friday when Canadian Vasek Pospisil withdrew with a back injury, giving him three full days off before his match against Robredo. Third-seeded David Ferrer beat No. 29 Jeremy Chardy 6-2, 7-6 (5), 6-2 and next plays Florian Mayer, who beat No. 7 Tomas Berdych beat Bosnian qualifier Damir Dzumhur 6-4, 6-2, 6-2 to set up a Sunday clash with No. 19 Kevin Anderson, who beat Edouard Roger-Vasselin 3-6, 4-6, 6-3, 7-6 (5), 7-5.