Serena Williams remained on track for her fifth Miami title with a 3-6 7-5 6-3 win over Svetlana Kuznetsova in their Sony Ericsson Open semi-final.

Kuznetsova won four straight games as she took command of the first set and was within sight of victory at 5-5 in the second.

But, despite requiring treatment for a back injury, Williams recovered to reach her sixth final at the event.

Fourth seed Jelena Jankovic plays Vera Zvonareva in the second semi-final.

Later on Thursday, Roger Federer plays Andy Roddick in the men's quarter-finals.

The winner will face Nikolay Davydenko, who thrashed Janko Tipsarevic 6-2 6-1.

Defending champion Williams made 43 unforced errors in just under three hours of action against Kuznetsova, with the intense heat a factor.

"I felt I made a lot of errors compared to the quarter-final (against Justine Henin)," said Williams.

"Svetlana played an entirely different game than I expected her to play, but once I got used to it I was okay."

The American started shakily and appeared to hurt her back chasing a backhand in the fourth game, prompting two visits from the trainer for court-side massages.

"It got better once the trainer came out," said Williams. "The first time she (came on the court), she did something that worked.

"It got stuck out of nowhere, but once she fixed it, it got better."

Kuznetsova looked the more likely to progress deep into the second set but she faltered when trailing 5-6, dumping an easy volley into the net at break point.

The final set opened with four straight breaks of serve but the key moment came when Williams broke for a 5-3 lead.

Kuznetsova said: "I think if I played the right points at the right time I could've won it in two sets.

"When I was on top of the game I played too safe. When she was on top, I was playing better.

"It's uncomfortable this situation because I thought I was playing well."

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