MIAMI, US - American Serena Williams survived a scare from China's Li Na to advance to the semi-finals of the ATP and WTA tournament where she awaits a possible tussle against her sister.

The top-seeded Williams lost the first five games but rallied to beat Li 4-6, 7-6 (7/1), 6-2 in the quarter-finals on Wednesday.

Ask to explain what went wrong, Serena said she couldn't get herself going, "I don't know. I just had a really slow, retarded start."

"She didn't give me any chance," Li said. "I had six or seven break points but I didn't hold for one point.

"In the final set she played much better. In the first set she didn't try 100 percent."

Williams is seeking her sixth Miami title, which would break the record she shares with Steffi Graf.

Three-time champion Venus Williams faces Agnieszka Radwanska in the last of the women's quarter-finals late Wednesday.

The other semi-final on Thursday will be between eighth-seeded Svetlana Kuznetsova and No. 11 Victoria Azarenka.

Serena said she would give herself a failing grade for the first set.

"If I was in school and I was like a professor and I had to give myself a grade, I would give myself like an eight percent out of 100."

Asked if she would remember this match and try to learn from her mistakes, Serena said, "I'm thinking about shredding the DVD too."

She said she usually talks more out loud when she is on her game and that didn't happen against Li.

"I wasn't as upbeat and spirited as I normally am. I think maybe one or two "come ons" during the whole match which is not really normal for me."

Serena said she expects to play better if he has to face Venus in the semi-finals.

"I am always going to be up for V," Serena said. "Because she's playing really well and always playing me super tough.

"It is like I automatically have to be on a different level because she's already playing a different level.

"Her balls are harder and her serve is way bigger. It is like I have to be there. And it is super fun to hit those serves that are like 120. It is frustrating but at the same time fun."

Li was the third Chinese player, Serena had defeated in a row. Serena also beat Zheng Jie 7-5, 5-7, 6-3 and Peng Shuai 7-5, 6-2.

"I was feeling like she play Fed Cup, three in a row Chinese players," Li said. "Good for the Chinese. They have more experience to play No. 1 in the world."