MELBOURNE--Ice-cool Roger Federer showed nerves of steel to down Croatian giant Ivo Karlovic on Friday as his great rival Rafael Nadal extended his hot streak at the Australian Open.

Denmark's Caroline Wozniacki reached the fourth round without dropping a set as she clings to the number one ranking, while Victoria Azarenka did not let a late display of petulance interrupt her progress.

With the 6'10" (208 cm) Karlovic hammering down serves and constantly coming forward, blocking the court with his outsized frame, Federer faced an awkward task.

But at set point down in the first set tie-break and scrambling to a drop shot, he did not flinch and gamely aimed a vertical lob over the Croatian beanpole, who could only parry it into his own court.

Federer, combating the fastest serve in the game with his unerring backhand, took his first set point then conjured the match's first break to take the second set and sauntered through the third to clinch it 7-6 (8/6), 7-5, 6-3.

"I knew it could come down to a few (points) here and there and obviously you need a bit of luck," he smiled. "I definitely got a bit fortunate and I started to play better as the match went on."

Earlier, Nadal bludgeoned his way into the fourth round with a straight-sets win over Slovakia's Lukas Lacko -- but he denied he was having it easy at the year's opening major.

Nadal came into the tournament jaded and troubled by injuries, but he has given no such signs on the court so far. After a hard-fought opening against Lacko, the 2009 champion raced away with it 6-2, 6-4, 6-2.

Wozniacki won 6-2, 6-2 against Monica Niculescu as she aims for her first major title and defends her top ranking from Azarenka, Maria Sharapova and Petra Kvitova, who could all be crowned number one at the end of this event.

The Dane, boosted by mental tips from her golf major-winning boyfriend, Rory McIlroy, took charge early against the Romanian and suffered her only lapse at 5-1 in the second set, when she was broken, before quickly recovering.

"Whatever happens, happens," Wozniacki said of the number one ranking.

Azarenka's temper flared when she wanted to contest a call on match point against Germany's Mona Barthel -- but she had run out of Hawk-Eye challenges, prompting a brief display of histrionics and opprobrium from the crowd.

After sealing it 6-2, 6-4 to stay unbeaten this year, the Belarusian third seed said she needed the extra emotion to get over the line.

And Andy Roddick was left to rue the latest in a series of late-career injuries which forced him to pull out of his veterans' battle against Lleyton Hewitt in Thursday's night match.