KEY BISCAYNE, Fla. — Serena Williams needs one more win for her fifth Key Biscayne title.

Williams overcame a slow start and sore back today to beat Svetlana Kuznetsova 3-6, 7-5, 6-3 in the semifinals of the Sony Ericsson Open.

The match took nearly three hours, and it turned into a test of stamina on a muggy, 85-degree afternoon. In the final set, Williams broke for a 5-3 lead when she skipped a backhand off the net cord, leaving Kuznetsova with an awkward backhand she dumped into the net.

Williams slammed her 12th ace to reach match point. Kuznetsova then sailed a lob long, and Williams raised her fist in weary triumph.

She improved to 46-5 at Key Biscayne, where she won titles in 2002, 2003, 2004 and 2007. In Saturday's final, the No. 8-seeded Williams will play the winner of the semifinal tonight between No. 4 Jelena Jankovic and No. 19 Vera Zvonareva.

Kuznetsova, the 2006 Key Biscayne champion, eliminated three-time champion Venus Williams in the quarterfinals and made a strong bid to complete a sweep of the family.

Serena Williams had a flurry of unforced errors at the start, and she appeared to hurt her back chasing a backhand on the final point of the fourth game. In the next game, Kuznetsova fell behind love-40 on her serve, then won five consecutive points for a 4-1 lead.

Williams then took an injury timeout, and a trainer probed and massaged her back. When play resumed, Williams' movement and serve appeared hampered for several games but gradually improved, and a 125-mph ace in the second set suggested her back was better.

With Kuznetsova serving at 5-6 in the second set, Williams broke for the first time since the opening game to even the match. She won the first three points with ferocious groundstrokes, and Kuznetsova blew an easy volley on set point.

Williams won despite 43 unforced errors, and she converted only five of 16 break-point chances.