updated 9:17 p.m. ET March 31, 2008

Lindsay Davenport hit a bump in her career comeback when she lost to Dinara Safina 6-3, 6-4 at the Sony Ericsson Open on Monday.

It was a flat followup to Davenport’s win Sunday over No. 2-ranked Ana Ivanovic. That upset victory was the biggest for Davenport since she returned to the women’s tour last summer after becoming a mother.

“That’s the thing with tennis,” Davenport said. “Every day you start over. But hopefully more positives than negatives come from this tournament.”

Positives came with the negatives for Serena Williams, too.

Williams always seems to rise to the challenge at the Sony Ericsson Open, where she has won four titles and is back in the quarterfinals. She beat Kaia Kanepi 6-3, 6-Monday and will next play top-ranked Justine Henin.

The No. 8-seeded Williams looked sloppy in a third-round victory, when she committed 60 unforced errors, but she pared that total to 24 against Kanepi. It was clear Williams meant business: She screamed “C’mon!” when she hit an ace, then shrieked when she a sloppy shot, and both times she could be heard in the top row of the stadium.

“I definitely feel like I was focused,” Williams said. “I felt like I had to be. The other match was definitely a good eye opener.”

She’s sure to be fired up again Tuesday when she faces Henin. Williams has lost their past three meetings, all last year, at the French Open, Wimbledon and the U.S. Open.

“We definitely bring out some of the best tennis in each other,” Williams said. “And she tends to play really well against me — probably better against me than other opponents. So it’s definitely a good match for me.”

No. 6 Venus Williams joined her sister in the quarterfinals by beating 17-year-old Caroline Wozniacki 6-3, 6-3. No. 1 Roger Federer and No. 6 Andy Roddick reached the fourth round on the men’s side.

Against Safina, Davenport totaled 34 unforced errors, including six double faults. She hit only 20 winners to 31 for Safina, seeded 13th.

“She did everything better than I did,” Davenport said. “I came up against a player today that was real intense and real eager to win.”

Federer, still seeking his first tournament title this year, advanced to the fourth round when Robin Soderling retired because of illness trailing 6-4, 3-0. Soderling lost the last five games and the final 10 points.

“You’re sort of happy you got through to the next round,” Federer said. “You’re not happy for your opponent, because it’s never a way you want to win a match, but you’ve got to push him to give up.”

Roddick converted only two of nine break-point chances but still beat qualifier Ivo Minar 7-6 (4), 6-4.

A superior serve and return helped Venus Williams defeat Wozniacki, a Dane who has won 14 matches this year to climb to a career-best 43rd in the rankings. Williams lost only six points on her first serve and repeatedly slammed Wozniacki’s serve for winners.

“I played some good tennis and aggressive when I needed to, and a lot of really good service games,” Williams said.

Williams finished with 23 winners and only 18 errors. She’s 10-4 this year and trying for her first 2008 title.

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Williams is a three-time Key Biscayne champion, but her most recent title in the event came in 2001.

“I definitely feel like I’m playing well,” she said. “I’m always trying to take it to another level, regardless. If I’m playing on cloud nine, I’m trying to get to cloud 10, and actually cloud 11.”

Henin hit eight aces, held every service game and beat Elena Vesnina 6-2, 6-2. No. 3 Svetlana Kuznetsova defeated No. 16 Shahar Peer 7-6 (4), 6-3. No. 4 Jelena Jankovic, who overcame five match points in the second round, swept Zheng Jie 6-4, 7-5.

In men’s play, No. 4 Nikolay Davydenko beat Simone Bolelli 6-7 (5), 7-5, 6-1.

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