Serena and Venus Williams to clash in semi-finals

World number one Serena Williams survived a scare to beat China's Li Na 4-6 7-6 6-2 on Wednesday night 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.

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, but found composure and ran out a comfortable winner to book a semi-final against Serena.

Venus has a 10-9 record against her sister but last 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.