MELBOURNE, Australia — Roger Federer improved his six-year winning streak to 15 matches against Lleyton Hewitt with an overpowering 6-2, 6-3, 6-4 victory in the fourth round at the Australian Open today.

The top-ranked Federer, beaten in last year's final at Melbourne Park by Rafael Nadal, hasn't lost to Hewitt since the Australian beat him in a Davis Cup match in the same Rod Laver Arena in late 2003.

The Swiss star has reached the semifinals or better here every year since winning the 2004 title, his first of three in Australia. He improved to 17-8 against Hewitt, and their 25 head-to-head matches is the most among active players.

Federer broke Hewitt's serve in the ninth game of the third set with a stinging cross-court forehand that left Hewitt standing in the middle of the court. Federer held service in his next game, setting up match point with an ace and clinching it with a service winner.

"When I saw the draw and saw Lleyton floating around, it's not something I like to see," Federer said.

As usual, Federer rose to the occasion.

"Hardly made any unforced errors," Federer said. For the record, he made 30 to go with his 49 winners.

"If there were some, they were at moments I can live with. I was really able to press on the offensive, serve well when I had to, and I moved well as well. Overall I'm extremely happy."

Hewitt agreed that Federer played "special" tennis.

"He hit the ball extremely clean, and I just couldn't get into his service games, which made it tough," Hewitt said.

The win advanced Federer to a quarterfinal against Nikolay Davydenko, who beat 2009 semifinalist Fernando Verdasco 6-2, 7-5, 4-6, 6-7 (5), 6-3 earlier Monday to extend his winning streak to 13.

In other matches, 2008 champion Novak Djokovic beat Poland's Lukasz Kubot 6-1, 6-2, 7-5 and Jo-Wilfried Tsonga of France, who lost to Djokovic in the 2008 final, beat Nicolas Almagro of Spain 6-3, 6-4, 4-6, 6-7 (6), 9-7.

Djokovic, who will play Tsonga in the quarterfinals, is hoping to end his drought in Grand Slam tournaments since he won his only major title at Melbourne Park.

"They are two different persons, physically I'm much stronger and able to get through the tough matches," the Serbian player said. "The 2008 Australian Open is by far the best tournament I ever played. ... I believe I can play the same this year."

Like Chris Evert, Martina Navratilova, Jennifer Capriati and Lindsay Davenport before them, Venus and Serena Williams did their part earlier Monday to ensure a 34-year-record at the Australian Open remained intact.

The Williams sisters won their fourth-round matches, extending a mark in which an American woman has been in the quarterfinals every year since 1977.

Venus Williams beat Francesca Schiavone 3-6, 6-2, 6-1 and Serena defeated Sam Stosur of Australia 6-4, 6-2 in just over an hour.

Navratilova, born in Czechoslovakia but who became an American citizen in 1981 — the same year she won her first Australian title — won here three times. Evert and Capriati captured it twice each and Davenport and Barbara Jordan once.

Monica Seles won the Australian Open four times, but only once as an American citizen — in 1996 — after she changed her citizenship officially from Yugoslavian.

Serena Williams has most recently had four wins at Melbourne Park, while Venus lost to Serena in the 2003 final.

Serena broke Stosur in the opening game of the second set Monday. The American faced two break points in her next service game, but fought back to deuce with an ace and a service winner, then served another ace on game point.

Stosur, who beat Serena the last time the pair met in California last year, set up break point in the seventh game with a double fault, then Serena hit a forehand winner to go up 5-2. Serena then held serve in the next game to take the match.

"It's important when you're playing a local girl to not let the crowd get too involved or else they'll kill you," Serena said.

Serena could add to her Grand Slam total, and Venus could win the Australian Open for the first time, but they can't meet in the final. They're in the same half of the draw and will meet in the semifinals if they win their next matches.

"I think it's challenging because we're both so good," Venus said of the Williams family matchups. "We both want to win. It's just difficult I think for us to play against each other because I think we have so much respect for each other's game."

Venus, who has won seven singles majors, struggled to hold serve in the first set against Schiavone but was in command by the end, when she won the last six games after the 29-year-old Italian broke her to open the third set.

Williams advanced to the Melbourne Park quarterfinals for the seventh time to set up a meeting with Li Na, who helped make Chinese history. For the first time at a major, there's two Chinese players in the quarterfinals.

Li, who beat No. 4-seeded Caroline Wozniacki 6-4, 6-3, joins Zheng Jie, the 2008 Wimbledon semifinalist, in the last eight. Serena Williams will play Victoria Azarenka of Belarus, who won the final 10 games in a 4-6, 6-4, 6-0 victory over Russian Vera Zvonareva.

Nadal takes on Andy Murray and Andy Roddick plays Marin Cilin in quarterfinals on Tuesday. Comeback player Justine Henin takes on Nadia Petrova — Henin leads the head-to-head matchups 12-2 — and Maria Kirilenko plays Zheng in women's quarterfinals.