KEY BISCAYNE, Florida (AP) -- Top-ranked Serena Williams twice lost five games in a row Monday but finished with a flourish and beat China's Zheng Jie 7-5, 5-7, 6-3 to reach the quarterfinals at the Sony Ericsson Open.

Serena Williams, seeking a record sixth Key Biscayne title and her third in a row, smacked an ace with her final swing, then raised a fist and began to pogo.

One, two, three, four times she hopped, showing uncommon exuberance over reaching the quarterfinals in a tournament she dominates. But survival is something to celebrate, and Williams barely made it out of the fourth round.

"I was just happy to get through it," Williams said, "because at one point I hit a ball into the bottom of the net and I thought, 'Wow, I haven't done this since I was a junior.' ... I wasn't playing great."

Also reaching the final eight was No. 5-seeded Venus Williams, who staged her own narrow escape to beat Agnieszka Radwanska 4-6, 6-1, 6-4. The Williams sisters could renew their sibling rivalry in the semifinals Thursday.

"I haven't seen any of Venus' matches here, so I should try to catch up on a few of those," Serena said. "I know she had a tough match today, and I followed suit."

Serena's reign was in danger when she fell behind love-2, 15-40 serving in the third set, but she rallied to take the lead, then swept the final eight points.

She won despite being broken six times and committing 43 unforced errors.

Venus' earlier match followed a similar script. She shanked overheads. She double-faulted three times in the final game. She grimaced and frowned and dropped her racket as if she wanted nothing more to do with it.

But the elder Williams picked up her racket -- and her game when it mattered most.

"Fortunately I've had experiences like that before," Venus said. "I know how to get through it."

The Williams sisters are the lone remaining players ranked in the top seven. Among those eliminated Monday were Olympic champion Elena Dementieva of Russia and two-time Grand Slam winner Amelie Mauresmo of France, who lost to unseeded Australian Samantha Stosur 6-4, 6-4.

On the men's side, where there have been no big surprises, top-ranked Rafael Nadal reached the fourth round by beating Portuguese qualifier Frederico Gil 7-5, 6-3. No. 4-seeded Andy Murray defeated Nicolas Massu of Chile 6-4, 6-4.

Venus Williams was playing for the third day in a row, and she looked weary in the warm, sunny weather.

"I was hitting really well in the practice," she said. "I came out and things weren't going the way I imagined they would. But that's tennis."

After dropping the first set, she began to move better and won 11 of the next 13 games.

The No. 10-seeded Radwanska nearly overcame a 5-1 deficit in the third set. In the final game Williams fell behind love-40, but erased three break points that would have made the score 5-all.

"Oh, that's always fun," Williams said. "Every now and then, everybody has those kinds of matches. ... I just had a lot of errors. Just errors, errors."

Williams finished with 40 unforced errors but also had 43 winners. She hit 11 aces, the last on the final point.

While Venus Williams is a three-time Key Biscayne champion, she hasn't won the title since 2001 and hasn't been to a semifinal since 2005.

"I feel good. I feel confident," she said. "I've been playing very well in the last few months."

Her quarterfinal opponent will be No. 26-seeded Iveta Benesova of Czech Republic, who swept No. 19 Anabel Medina Garrigues of Spain 6-4, 6-1.

The No. 4-seeded Dementieva lost to 18-year-old Caroline Wozniacki 7-5, 6-4. Wozniacki, the first Danish woman to crack the top 20, has reached a career-high 12th in the rankings while benefiting from recent workouts in Las Vegas with Andre Agassi's former trainer, Gil Reyes.

"It feels amazing to me that I'm the No. 12 in the world," she said. "As a little girl growing up I was always watching everyone on TV, the top 20 players, and I wanted to be that girl as well."

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