MELBOURNE, Australia -- No. 3-ranked Novak Djokovic advanced to the fourth round of the Australian Open after playing just one set Friday against Serbian Davis Cup teammate Viktor Troicki. The 2008 Australian Open champion had just won the first set 6-2 when 29th-seeded Troicki retired, telling the chair umpire he couldn't continue because he had a stomach muscle strain. He'd spoken briefly to a trainer during a change of ends after Djokovic earned the first service break to go ahead 3-1. "It's not really the way you want to win - especially if you're playing against one of your best friends," Djokovic said. "Unfortunately he wasn't able to play 100 percent of his abilities." The pair spent a long time celebrating after Serbia's first ever Davis Cup title, which Troicki clinched 3-2 with a win in the deciding singles match last month. "Viktor was actually the hero. He won the last match. He played the best match of his life there," Djokovic said. "We're still under a lot of emotions. A lot of celebrations that I wouldn't talk about publicly!" Rafael Nadal has had plenty to celebrate after three consecutive Grand Slam titles. His winning streak in majors is up to 23 matches and he's five wins from being the first man since Rod Laver in 1969 to hold all four Grand Slam titles at once. Nadal pounded another up-and-comer into submission on Thursday, beating American qualifier Ryan Sweeting 6-2, 6-1, 6-1. Andy Murray made quick work of his second-round opponent as well. And Kim Clijsters just wants everyone to know she's not pregnant again. She brought up the subject herself - right there on the stadium court before an interviewer - and swatted down the misguided chatter with great delight as if putting away an opponent with an overhead. Clijsters has already quit tennis once to get married and have a child, only to return after 2 1/2 years and win the 2009 U.S. Open. She won the U.S. Open again last year and is fast emerging as the Melbourne favorite in a draw missing injured defending champion Serena Williams. She's dropped only four games in two rounds, a 6-1, 6-3 win over Carla Suarez Navarro following her first-round 6-0, 6-0 romp over formerly top-ranked Dinara Safina. Nadal also has lost only four games in two rounds, and says his nine Grand Slam crowns have given him "a lot of calm" as he chases his Rafa Slam. "I have probably less pressure right now, last year's season, winning a lot," he said. "I have to work hard every day. That's what I do: keep practicing with ... motivation for sure, waiting my chances, trying to find my chances." Clijsters made the most of her chance with on-court interviewer Todd Woodbridge. The doubles great had sent a text message to fellow Australian Rennae Stubbs saying in not-so-subtle terms that Clijsters was acting and looking as if she's pregnant. Problem was, Stubbs showed Clijsters. Lucky all three are friends. "You thought I was pregnant?" she chided Woodbridge, laughing and patting him on the shoulder. "No, I'm not." Clijsters is ranked No. 3 and relaxed about her growing status as favorite. No. 2 Vera Zvonareva has reached the last two Grand Slam finals, but yet to win a major. She showed some jitters early before recovering to beat Bojana Jovanovski 2-6, 6-3, 6-1. No. 10 Shahar Peer and No. 12 Agnieszka Radwanska advanced along with No. 13 Nadia Petrova, who beat Clijsters 6-0, 6-1 in a big upset last year and could meet the Belgian again in the fourth round. Former No. 1 Jelena Jankovic extended a sorry run, losing to China's Peng Shuai for her seventh loss in eight matches. French Open finalist Samantha Stosur advanced by winning her night match 6-3, 6-2 over Vera Dushevina. She's one of two Australians still in the tournament. The other, 18-year-old wild card Bernard Tomic, advanced 7-6 (4), 7-6 (3), 6-3 over No. 31-seeded Feliciano Lopez. He plays Nadal in the next round. Fourth-seeded Robin Soderling advanced with a 6-3, 7-6 (1), 6-1 win over Gilles Muller. Murray, the losing finalist last year, beat Ukraine's Illya Marchenko 6-1, 6-3, 6-3 at Margaret Court Arena. No. 10 Mikhail Youzhny, No. 13 Jo-Wilfried Tsonga, No. 15 Marin Cilic and No. 32 Guillermo Garcia-Lopez all won. There are three Americans into the third round, with No. 20 John Isner winning Thursday. No. 8 Andy Roddick is playing Friday, with Venus Williams in action in the women's draw against Andrea Petkovic.