WTA Sony Open: Serena Williams battles past Li Na to reach last four in Miami

Serena Williams remains on course for a sixth Sony Open title after defeating Li Na 6-3 7-6 (7/5) to reach the semi-finals on Tuesday.

Williams, who has now levelled Steffi Graf's record of 59 winning matches at the event, battled back from 5-2 down in the second set to clinch a straight-sets win.

Breaking Li three times in each set, Williams surrendered her own serve on three occasions to win after one hour and 50 minutes. The powerful American fired down three aces and coughed up six double faults.

Williams now boasts a 7-1 head-to-head record against Chinese star Li, whose only win came at Stuttgart in 2008, but she admitted it was not easy.

"It's good to be able to at least come back," Williams said. "I like to believe that I try to be a solver. I just try to do things different if something is not working. She played such a good tie-breaker. It was important for me to win that.

"She has such fight. She never quits and I think that we have a lot of that similar."

Williams did admit to her serving problems though, and was sick of hitting so many double faults after another six against Li.

"I just can't hit any more double faults. It's embarrassing and unprofessional. I hit about 50 in one game and it was just outrageous. It was like at this point I shouldn't be a professional tennis player.

"That was my goal - I'm not hitting any more double faults."

Williams will face defending champion Agnieszka Radwanska in the last four after the fourth seed came back from a set down to claim a 4-6 6-4 6-2 win over Kirsten Flipkens.