World number one Serena Williams survived a scare to beat China's Li Na 4-6, 7-6, 6-2 on Wednesday and reach the Sony Ericsson Open semi-finals -- where she will meet her sister Venus.
Venus beat the Czech Republic's Iveta Benesova 6-1, 6-4 to ensure that at least one of the sisters will be in the final at Key Biscayne.
Serena made an awful start and trailed 5-1 in the first set following a string of unforced errors before Li won it 6-4.
Neither player could break serve in the second set although Serena came close to blowing a game at 5-5 when she twice double-faulted.
The American dominated the tiebreak, however, which seemed to rock Li's confidence.
Williams broke Li's serve at the start of the third set and from then on victory became a formality as the Chinese handed her opponent several easy points.
Williams was puzzled by her slow start.
"I just had a really slow start. Maybe I was tired, but that's still not an excuse really," she told reporters. "I definitely wasn't moving my feet at all. I just wasn't doing anything that made a lot of sense at all.
"I'm a little disappointed. I don't think I played great, but I'm glad to have gotten through."
Li, ranked 40th, said there was little she could do once Williams had begun to take control.
"She played better in the final set. After she won the second set I didn't think I had a chance because, I was feeling like she had stood up," she said.
In contrast, big sister Venus started confidently in the first set, breaking twice early before finishing off the set with a rocketing ace.
She took her foot off the gas in the second, allowing Benesova to go 3-0 up and then giving her three break points in the fourth game.
But she found her composure and ran out a comfortable winner to book a semi-final against Serena on Thursday.
Venus has a 10-9 record against her sister but earlier this month lost to her in an exhibition match at Madison Square Garden.
With the pair being similar in style and approach to the game, Venus said the outcome would come down to temperament.
"Our core beliefs and our core (play) is very similar. The difference in our game would be down to personality, I think," she said.
Serena said she expected to raise her game for her sister.
"I'm always going to be up for V, because she's playing really well and is always playing me super tough.
"Her balls are harder and her serve is way bigger ... It's frustrating, but at the same time fun," she said.