Serena Williams beats sister Venus at Key Biscayne

Key Biscayne: Serena Williams retained the No 1 world ranking by being her sister Venus 6-4, 3-6, 6-3 in the semifinals of the Sony Ericsson Open on Thursday.

The match was the 20th meeting between the sisters, and each has won 10 times.

Serena needed to reach the final to retain the top ranking she has held since Feb. 2. Otherwise she would have been supplanted next week by No. 2-ranked Dinara Safina of Russia.

"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."

Serena's opponent on Saturday will be 19-year-old Victoria Azarenka of Belarus, who beat 2006 champion Svetlana Kuznetsova 6-3, 2-6, 7-5. Azarenka became ill with a stomach complaint before losing a fourth-round match against Williams at the Australian Open in January.

Serena jumped to a 4-1 lead in the final set against her older sister and broke serve for the sixth time in the final 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.

"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 or not."

Top-ranked 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.

Del Potro had lost all nine sets in their four previous matches, but he was buoyed by a home-court advantage, with many transplanted Argentines in the crowd singing "Del-Po" between points.

"Wonderful for the crowd," top-ranked Nadal said. "Terrible for me."

The No. 6-seeded del Potro's opponent on Friday will be No. 4 Andy Murray of Britain, who beat No. 8 Fernando Verdasco 6-1, 6-2. Murray broke serve five times and saved seven of eight break points against him.

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.

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.

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

Serena is seeking her sixth Key Biscayne title, which would break the record she shares with Steffi Graf. She's 52-5 in the event, with one loss since 2001 — to Venus.

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.

Del Potro showed patience in long rallies and used his looping forehand to keep Nadal deep as their three-hour match built to a dramatic finish.

"I beat him with my mind and with my game," del Potro said. "When we played long points, I was dominating every time."

Nadal was up two breaks in the final set at 3-love, but del Potro climbed back into the match by winning 12 of the next 14 points.

"I played really bad all the time," Nadal said. "When I have it 3-love in the third, I played worse. It was amazing disaster."

When Nadal lost serve for the second consecutive time for 3-all, del Potro let loose a primal scream. There was more noise to come.

In the 12th game of the set, Nadal saved three match points, two with aces. He caught a break in the tiebreaker when his return skipped along the net cord before clearing it for a winner and a 3-2 lead.

That was the last point Nadal won. Del Potro hit three consecutive winners to go up 6-3, and on the final point Nadal put a backhand in the net as the crowd erupted one last time.

"I beat the No. 1," del Potro said. "If you don't play unbelievable, you cannot beat him."

Nadal, a six-time Grand Slam champion, won at Indian Wells two weeks ago, but his best finish at Key Biscayne was as the runner-up in 2005 and 2008.

"I didn't play well during this tournament," Nadal said. "I didn't adapt well. I played really bad."

Azarenka closed out her victory on her fourth match point when Kuznetsova netted a backhand to end the 2-hour, 40-minute endurance test played in 85-degree F (30-degree C) sunshine.

"It's the first final in such a big tournament for me," Azarenka said. "It's the biggest moment, I would say, in my career. It's all kind of an experience for me. It's a new thing that I'm going to be introduced to on how to handle."