TENNIS in DEPTH.

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, said he’s playing well despite having a dry run at Grand Slam tournaments since his Melbourne Park victory.

“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.”

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.

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 upset No. 4-seeded Caroline Wozniacki 6-4, 6-3, joins Zheng Jie, the 2008 Wimbledon semifinalist, in the last eight.

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.

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