Sony Ericsson Open, Miami
25 March to 5 April
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Williams is closing in on her sixth title at Key Biscayne
Serena Williams will take on her sister Venus in the semi-finals of the Sony Ericsson Open after the top seed survived a scare on Wednesday.
The world number one recovered from losing the opening set against Li Na to come through 4-6 7-6 (7-1) 6-2.
Venus Williams, the fifth seed, had an easier time against Czech Iveta Beneseova as she won 6-1 6-4 in Miami.
"I believe we are the best in the world," Venus said, looking ahead to her 20th match against her sister.
"I enjoy playing Serena because we challenge each other the most."
Venus - a three-time winner in Miami - leads their personal battle 10-9, having won the last two matches between the pair.
Playing her, it's like I have to automatically be on a different level
Another victory for Venus would see her sister lose the world number one ranking to Russian Dinara Safina.
Five-time and defending champion Serena said: "I'm always going to be up for 'V,' because she's playing really well and always playing me super tough.
"Playing her, it's like I have to automatically be on a different level, because she's already playing on a different level.
"Her balls are harder and her serve is way bigger. And it's super fun to hit these serves that are like 120 mph. It's frustrating but at the same time fun."
Thursday's other semi-final will be between eighth seed Svetlana Kuznetsova and 11th seed Victoria Azarenka.
In the first of Wednesday's quarter-finals, Serena conceded the first five games in a sloppy opening strewn with mistakes. She then saved six break points in a tightly-fought second set as Li forced her to dig deep to stay in the match.
But the American found another gear to take a stranglehold on the tie-break and break Li's first service game in the decider.
The match completed its decisive swing to the favourite as the defending champion punished several loose groundstroke and Li crumbled.
"Maybe I was tired, but that's still not an excuse really," said Serena. "I definitely wasn't moving my feet at all. I just wasn't doing anything that made a lot of sense at all."