PHOTOS: Nadal and Federer shine, Clijsters cruises

Victoria Azarenka needed to get her emotions flowing to close out her 6-2, 6-4 victory over Mona Barthel and reach the fourth round of the Australian Open on Friday.

The third seed came through a strong field to win the title at last week's Sydney warm-up tournament and establish herself as the in-form contender for the year's first grand slam.

German Barthel at least stretched the Belarussian more than her opponents in the first two rounds, where she enjoyed 6-1, 6-0 wins, and it could have been a tougher test if the 21-year-old had capitalised on three break points.

The rangy Barthel, who won a warm-up event in Hobart as a qualifier and was making her debut at Melbourne Park, battled right until the end to leave Azarenka, out of challenges and unhappy with a few line calls, fuming in the final game.

Whereas once she might have fallen apart, Azarenka channeled the emotion into her game and clinched her place in the last 16 on her fifth match point when her opponent netted a volley.

"I had to get a little bit... not angry like in a bad way, just get my emotions going a little bit to finish the match, because I had few chances and I didn't convert them," she said.

"Sometimes you just have to push yourself a little bit to get you going again."

Azarenka said she sometimes struggled to contain her "bad emotions" but that her days of melting down on court were largely behind her. 

Rafa Nadal sent the last qualifier spinning out of the Australian Open on Friday with a comfortable 6-2, 6-4, 6-2 win over Slovakian Lukas Lacko.

Nadal could not have asked for smoother passage into the fourth round, wrapping up victory in an hour and 55 minutes in overcast conditions on Rod Laver Arena and conserving valuable energy for the wars of attrition to come in later rounds.

The world number two came into the year's first grand slam with injury clouds hanging over his head, but the Spaniard has yet to lose a set at Melbourne Park and on Friday there was little to suggest his knee or shoulder were bothering him.

"The knee is fine, that's the important thing," the 2009 champion said in a news conference.

"The match was a really complete match, a really solid one. I'm very happy about my game. Being in the fourth round without losing a set, it's fantastic news."

Despite cruising through three seemingly simple matches so far, Nadal felt he had been tested but the level of his play had made things look easy.

"I am playing well. If you are playing well, things can be a little bit less difficult. If you are playing bad, every match will be very, very difficult," he added.

Top seed Caroline Wozniacki continued her quest for a maiden grand slam title despite some trouble closing out her 6-2, 6-2 victory over Romania's Monica Niculescu in the third round of the Australian Open on Friday.

The 21-year-old world number one was broken by the 31st seed when she was serving for the match at 5-1 but the hard work had already been done under overcast skies on Hisense Arena.

She wasted no time in breaking back immediately to clinch the victory with a rasping backhand winner after 76 minutes and reach the fourth round without dropping a set.

The Dane will play Christina McHale or 13th seed Jelena Jankovic in the fourth round. 

Roger Federer was given just the work out he needed after getting a walkover when he beat the big-serving Croat Ivo Karlovic 7-6, 7-5, 6-3 on Rod Laver Arena on Friday to advance to the Australian Open fourth round.

The third-seeded Swiss had easily accounted for Russian qualifier Alexander Kudryavtsev in 98 minutes in the first round then had a walkover in the second following Andreas Beck's withdrawal with a lower back injury.

Karlovic's 2.08 metres (6-foot-10) height created odd service angles for Federer, but the 16-times grand slam winner used his court craft and foot speed to punch returns down the line or cross court and seize what break opportunities he could. 

Champion Kim Clijsters made up for her Brisbane International semi-final disappointment against Daniela Hantuchova earlier this month with a polished 6-3, 6-2 victory over the Slovak in the third round.

The 28-year-old Clijsters quit with a hip injury while a set ahead against Hantuchova in Brisbane but said prior to the Australian Open began that it had healed completely and would not hamper the defence of her title.

Clijsters had spent less than two hours in total on court in the first two rounds at Melbourne Park and continued her ruthlessly efficient progress through the tournament against the 20th seed, converting five of 11 break point opportunities to seal a comfortable win.

Spain's Feliciano Lopez beat John Isner 6-3, 6-7, 6-4, 6-7, 6-1 on Hisense Arena to leave the United States without a single man standing in the last 16 of the Australian Open for the first time since 1973.

"It's very disappointing," Isner told reporters. "That's not a good effort from the Americans this tournament. And I knew going in today I was the last one left and I wanted to keep on going, but just didn't happen.

"But it's very ugly, to be honest, to have no one in the round of 16. We've got to try to rectify that next time the big tournaments roll around."

Eleven American men played in the first round earlier this week with six, including seeds Isner, Mardy Fish and Andy Roddick, making it through to the second round.

There were no American men in the fourth round in 1972 and 1973 because none entered the tournament. Otherwise there has always been at least one player from the US in the men's last 16 since tennis went professional in 1968.

Isner's exit marks a new low for the US, who last year were left without a representative in the men's or women's quarter-finals for the first time since 1987, when not all top players made the trip Down Under.