KEY BISCAYNE, Fla. — Despite a sluggish start and wobbly finish, Venus Williams advanced Monday to the quarterfinals at the Sony Ericsson Open.

The three-time champion double-faulted three times in the last game before finishing with an ace to beat No. 10-seeded Agnieszka Radwanska 4-6, 6-1, 6-4.

Williams, seeded fifth, avoided the wave of upsets that has eliminated six of the top 10 women. She and her sister, top-ranked Serena Williams, are the lone remaining players in the top seven. The sisters could renew their rivalry in the semifinals Thursday.

On the men's side, where there have been no big upsets, top-ranked Rafael Nadal reached the fourth round by beating qualifier Frederico Gil 7-5, 6-3.

Williams was playing for the third day in a row, and she looked weary in the warm, sunny weather. After dropping the first set, she began to move better and won 11 of the next 13 games.

Radwanska nearly overcame a 5-1 deficit in the final set, but she failed to convert three break points that would have made the score 5-all.

Williams finished with 40 unforced errors but also had 43 winners, hitting 11 aces. She's seeking her first Key Biscayne title since 2001.

No. 4-seeded Elena Dementieva lost her fourth-round match to 18-year-old Caroline Wozniacki 7-5, 6-4. Two-time Grand Slam champion Amelie Mauresmo was eliminated by unseeded Samantha Stosur 6-4, 6-4. No. 8 Svetlana Kuznetsova beat Alisa Kleybanova 6-2, 6-1.