MELBOURNE, Australia -- Serena Williams is still trying to keep up with big sister Venus.

After beating Els Callens 6-4, 6-0 Thursday in the second round of the Australian Open, Serena admitted Venus' one-sided victory over Ansley Cargill the previous night inspired her.

Day 4 highlights

Weather: Overcast, turning to a sunny day, high of 75. Attendance: 51,413 (14,476, night). Stat of the day: 14. The matches Kim Clijsters has won consecutively since October. She beat Petra Mandula of Hungary 6-0, 6-0 in 33 minutes Thursday. Quote of the day: "We're in 16 of January, man. I don't know if I will be alive tomorrow." -- Marat Safin, when asked if he would be playing in Beijing later this year.

"She played a great match, I was motivated after watching her, thinking, 'OK, she wants to be No. 1 again,' " said Serena, seeking her fourth straight Grand Slam title.

The top-ranked American's opener against Emilie Loit was too close for comfort. The Frenchwoman took her to three sets, pounding her with an aggressive forehand and frustrating her so much that she groaned an obscenity. She was warned for it and fined $1,500 Thursday.

Serena jumped to No. 1 in the world with victories in the French Open, Wimbledon and the U.S. Open, beating Venus in all three championship matches to set up a shot at the "Serena Slam."

"I have five more matches to go. It's going to be really tough," Williams said.

Serena was not around for the second round last year. An ankle injury forced her to withdraw out on the eve of the season-opening major.

Also Thursday, an ankle injury cost Monica Seles a place in the third round. Seles, seeded sixth, fell to Grand Slam rookie Klara Koukalova of the Czech Republic 6-7 (6), 7-5, 6-3.

Seles, a four-time champion at Melbourne Park, rolled her left ankle attempting to change directions in the third game, trailing 0-30 and at a game apiece. She dropped serve in that game, after receiving attention on the court and on the sideline, but rallied to win the set.

She countered Koukalova's efforts to run her round the court, relying on her big serve and powerful returns to stay in the match. In the end, it was a deft drop shot on match point that Seles couldn't reach.

"I was in bad pain. It was an ankle sprain," she said. "I struggled from the second game on. I was just a step slower and I couldn't change direction. I tried to tough it out, but I couldn't."

Belgium's Kim Clijsters needed just 33 minutes for her 6-0, 6-0 victory over Petra Mandula, giving her the first "double bagel" of the tournament.

The fourth-seeded Clijsters beat Venus and Serena Williams en route to the WTA Championships title in November and is seeded to meet Serena in the semifinals. Clijsters has lost only three games in the first two rounds, and has won 21 of her last 22 matches.

Clijsters conceded just 17 points to Mandula, who suffered the humiliation of becoming only the 13th woman to fail to win a single game in a completed match at the Australian Open.

"I think she took it well," said Clijsters, who also inflicted a 6-0, 6-0 whipping on Marta Marrero at the 2000 U.S. Open. "She was still smiling, so that's a good sign.

Against Callens, ranked 66th, Serena Williams saved two break points on her first service game before breaking in the fifth. She won 56 points against 36 for Callens, producing 21 winners and reducing her unforced errors to 13.

Her next opponent is No. 26 Tamarine Tanasugarn of Thailand, a 6-2, 3-6, 6-3 winner over Russian Alina Jidkova.

Serena mentioned No. 10 Chanda Rubin, along with Clijsters, as contenders for the Australian title.

Rubin immediately lost every game in her first set against Mary Pierce but rallied for a 0-6, 6-4, 6-2 win.

Rubin, who missed the first four months of 2002 because of knee surgery, needed her right ankle taped during the match. Pierce, the 1995 Australian and 2000 French champion, struggled with blisters and needed a timeout.

"I felt like I was out of it in the first set, it was over so quickly. She hits the ball so big and has punishing strokes," Rubin said. "It was definitely a tense match for me ... it was a matter of staying on two feet and getting over it."

No. 8 Anastasia Myskina beat Swiss player Emmanuelle Gagliardi 5-7, 6-2, 6-0, and No. 11 Magdalena Maleeva beat South Korea's Cho Yoon-jeong 2-6, 6-4, 6-1.