KEY BISCAYNE, Fla. — 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. Otherwise she would have been supplanted next week by No. 2-ranked Dinara Safina.

Serena's opponent Saturday will be 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 jumped to a 4-1 lead in the final set against Venus 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.

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.

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

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

"They helped me a lot," del Potro said.

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. Murray broke serve five times and saved seven of eight break points against him.

The 6-foot-6 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 is a six-time Grand Slam champion, and he 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 kept waving clenched fists during her semifinal, as if there was any doubt she meant business. Celebrating every important point she won, the teenager earned her biggest victory yet.

"It's the first final in such a big tournament for me," the No. 11-seeded Azarenka said. "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."

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

"I'm just so happy that I found energy in that fighting moment," Azarenka said. "That was kind of adrenaline, I would say, on the last games."

Azarenka, who trains in Scottsdale, Ariz., is ranked a career-best 10th after reaching the semifinals at Indian Wells two weeks ago.