MELBOURNE, Australia (AP) — Roger Federer made the most of his rare opportunities Friday against the fastest serve in tennis, cashing in with some classic returns in a 7-6 (6), 7-5, 6-3 win over Ivo Karlovic to reach the fourth round of the Australian Open.

The four-time Australian Open champion fended off Karlovic's set point in the tiebreaker with a return that brought the 6-foot-10 Croatian to the net, then lobbed just over him.

"I knew going in it was going to be tough. I played him 10 times, and we've played some breakers. I knew it could come down to a few here and there," said Federer, who broke Karlovic once in each of the second and third sets. He only faced two breakpoints himself, including the pivotal one in the opening tiebreaker. "I definitely got a bit fortunate and started to play better as the match went on."

For his part, Karlovic thought it was "one in a 100 I'm gonna lose that point."

"It was unlucky," Karlovic said. "I didn't really expect him to do that. I was there, I just miscalculated how much I was jumping. If I would have won that, everything would be different but that's life."

Federer is now 10-1 against Karlovic and looking increasingly confident at Melbourne Park, where he has collected four of his 16 Grand Slam titles.

He and Rafael Nadal are on the same side of the draw at a major for the first time since 2005 and could meet in the semifinals.

Second-ranked Nadal had a 6-2, 6-4, 6-2 win earlier Friday over Slovakia's Lukas Lacko to advance without dropping a set or show any signs that a freak knee injury is bothering him.

Nadal, who won the 2009 Australian title but has gone out in the quarterfinals due to injuries in the last two years, felt a crack and then sharp pain in his right knee while sitting in a chair at his hotel on the weekend and was concerned that he might not be able to play in his opening match. Medical tests didn't show any serious damage, and he has had the knee heavily taped in his three matches since.

"The knee is fine. That's important thing," the 10-time major winner said. "The match was a really complete match, a really solid one.

"Very happy about my game. Being in fourth round without losing a set, it's fantastic news."

He will play either John Isner or Feliciano Lopez in the next round.

On the women's side, top-seeded Caroline Wozniacki continued her quest for a first major title with a 6-2, 6-2 win over Monica Niculescu of Romania and third-seeded Victoria Azarenka beat Mona Barthel 6-2, 6-4 in a match between champions of two warmup tournaments.

No. 8 Agnieszka Radwanska beat Kazakhstan's Galina Voskoboeva 6-2, 6-2 and will next meet No. 22 Julia Gorges, who beat Italian Romina Oprandi 3-6, 6-3, 6-1.

Wozniacki, who needs to reach the quarterfinals to have any chance of retaining the No. 1 ranking, wasted one match point and was broken when she was serving for the match, but broke back immediately to ensure she moved into the Round of 16.

Azarenka, who beat French Open champion Li Na to win the Sydney International last week, has only lost eight games at Melbourne Park and remains one of three women who can overhaul Wozniacki for the top ranking at the Australian Open.

The 22-year-old from Belarus will next meet Czech player Iveta Benesova, who beat Russian qualifier Nina Bratchikova 6-1, 6-3.

Barthel was on a 10-match winning run in Australia after capturing her first title at the Hobart International last week as a qualifier.

Barthel hit 20 winners — one more than Azarenka — but she was broken three times and failed to convert three break opportunities.

Azarenka was annoyed with herself for needing five match points to finish off Barthel, and for running out of challenges before she really needed to review a line call in the last game.

"I've been playing in the end not brave enough to finish the match ... I had to get a little," angry, Azarenka said.

Federer will play the winner of the match between Australian teenager Bernard Tomic, a Wimbledon quarterfinalist last year, and No. 13 Alex Dolgopolov. Both have a vastly different style to Karlovic, who broke Andy Roddick's record for the fastest serve ever in tennis by hitting one at 156 mph in a Davis Cup match last year.

"It's just because he's so tall and makes it unusual to return against," Federer said of the serve. "I've been around the block, faced some good ones."

Yet it unraveled twice when Karlovic needed it, in the tiebreaker and in the 12th game of the second set.

Karlovic was a point from forcing a second-set tiebreaker but Federer stepped it up, earning two set points with some trademark backhands and converting on the second when the tall Croatian netted a volley.

Roddick is already out of the tournament, retiring during his second-round match against Australian veteran Lleyton Hewitt late Thursday.

He needed a medical timeout after injuring his right hamstring in the second set and played 16 more games before finally retiring when Hewitt gained a 3-6, 6-3, 6-4 lead.

"It's a miserable, terrible thing being out there compromised like that," said Roddick, who is hoping to return within three weeks.

Hewitt, who turns 31 next month, goes to the third round against Milos Raonic, the big-serving, 21-year-old Canadian. If Hewitt eliminates an opponent who has dropped only two service games this year, he could face defending champion Novak Djokovic in the fourth round. Djokovic, who won three of the four major titles last year, kept getting better in his 6-3, 6-2, 6-1 win over Santiago Giraldo.

Fourth-seeded Andy Murray, who lost to Djokovic in last year's Australian final, ousted Edouard Roger-Vasselin of France 6-1, 6-4, 6-4. No. 5 David Ferrer beat American Ryan Sweeting 6-7 (4), 6-2, 3-6, 6-2, 6-3, and No. 6 Jo-Wilfried Tsonga downed Ricardo Mello of Brazil 7-5, 6-4, 6-4.

Five-time Australian Open champion winner Serena Williams notched her 500th career singles victory Thursday when she beat Barbora Zahlavova Strycova 6-0, 6-4 in the second round.

"Five hundred is a lot of matches to play, let alone to win," she said, adding that the left ankle she badly sprained two weeks ago wasn't an issue. "It's totally fine. It was my good ankle, so I'm good."

Williams won the Australian Open in 2009 and 2010, but didn't defend her title in 2011 because she was injured.