MIAMI, (Reuters) – World number one Serena Williams  survived a major scare to beat China’s Li Na 4-6 7-6 6-2 and  reach the Sony Ericsson Open semi-finals yesterday.

Williams, seeking her sixth title at Key Biscayne, could now  face her sister in the last four if Venus can overcome Czech  26th seed Iveta Beneseova.

Serena made an awful start and trailed 5-1 in the first set  following a series of unforced errors.

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, though, and the  convincing nature of that victory seemed to knock the confidence  out of the previously bright and positive Li.

Williams broke Li’s first serve in 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, retarded 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,” she said.

“I’m a little disappointed. I don’t think I played great,  but I’m glad to have gotten through,” added Williams.

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.