serena_williams_1.jpgKEY BISCAYNE (AP) — Serena Williams retained the No. 1 world ranking by being the best player in her family Thursday night.

Williams defeated older sister Venus 6-4, 3-6, 6-3 in the semifinals of the Sony Ericsson Open. It was their 20th sibling showdown, and each has won 10 times.

Serena needed to reach the final to retain the top ranking she has held since Feb. 2.

"I'm excited," Serena said. "I was thinking I'd love to remain No. 1. I think I was more happy about that than winning the match."

If Venus had won, Serena would have been supplanted in next week's rankings by No. 2 Dinara Safina.

"Even though she's my sister, I'm still here to win," Venus said. "I can't give anyone anything. So I'm disappointed that I lost tonight, whether or not she kept the ranking."

No. 1 Rafael Nadal lost in the men's quarterfinals to 20-year-old Argentine Juan Martin del Potro, 6-4, 3-6, 7-6 (3). The upset delighted a partisan crowd that included many transplants from Argentina.

"Wonderful for the crowd," Nadal said. "Terrible for me."

The No. 6-seeded del Potro's opponent Friday will be No. 4 Andy Murray, who beat No. 8 Fernando Verdasco 6-1, 6-2.

Serena Williams will try for a record sixth Key Biscayne title Saturday against 19-year-old Victoria Azarenka of Belarus, who beat Svetlana Kuznetsova 6-3, 2-6, 7-5. Azarenka became sick to her stomach before losing a fourth-round match against Williams at the Australian Open in January.

Serena raced to a 4-1 lead in the final set and broke serve for the sixth time in the last game. When she closed out her victory, she hopped in delight, raised a fist and shouted "Yes!" She then met her sister at the net with a handshake and slap on the back.

"It never gets easy," Serena said. "She's the toughest player I think on the tour, besides me of course."

As often happens, Williams vs. Williams was an aesthetic disappointment. Both players repeatedly went for winners, which resulted in many errors and few long rallies. Fans were subdued, with the majority heading back to the mainland by the third set.

The players were subdued, too, wearing impassive expressions and refraining from any demonstrative reactions to points – until the final game.

With Venus serving at 3-5, Serena hit a backhand winner and punched the air, then repeated the sequence five points later to reach match point.

The sisters say they're close, and the match offered evidence. Serena won 82 points, Venus 81. Serena had 38 unforced errors, Venus 40. Serena hit 19 winners, Venus 22.

One big difference: Venus twice double-faulted on break point.

"It was a well-fought match," Venus said. "My serve wasn't going as well as I wanted. But she brought a lot of balls back and played tough."

Venus, a three-time Key Biscayne champion, fell to 15-2 this year. Serena has won the tournament the past two years and is 52-5 in the event, with one loss since 2001 – to her sister.

After losing her past two meetings with Venus, Serena said she was glad to pull even in the rivalry.

"I would love to be in the lead again," she said.

Pictured above is Serena Williams.