KEY BISCAYNE, Fla. — Serena Williams lost the first five games Wednesday, then rallied to reach the semifinals at the Sony Ericsson Open, where she awaited a possible matchup against her sister.

The top-ranked Williams overcame a dismal start to beat Li Na 4-6, 7-6 (1), 6-2 in the quarterfinals. Williams seeks her sixth Key Biscayne title, which would break the record she shares with Steffi Graf.

Three-time champion Venus Williams was to face No. 26-seeded Iveta Benesova in the last of the quarterfinals Wednesday night. The other semifinal Thursday will be between No. 8-seeded Svetlana Kuznetsova and No. 11 Victoria Azarenka.

Serena Williams was on the verge of being upset, but in the pivotal second set, the unseeded Li failed to convert all six of her break-point chances. The last came at 5-all, and Williams erased it with a service winner.

Li made things easy for Williams in the tiebreaker by committing six unforced errors. As the third set began to slip away from Li, she showed her frustration by slamming a ball against the backstop on the far end of the court.

She then double-faulted on break point to fall behind 2-1. Williams won the next three games at love and closed out the victory with a service winner.

The finish was in stark contrast to the start, when Williams looked lethargic in the 80-degree sunshine. Several times she declined to chase after Li's shot.

"Wake up!" one spectator shouted.

It was part of a familiar pattern of inconsistency for Williams, who has managed the nifty feat of reaching the semifinals despite losing five consecutive games three times in her past two matches.

She improved her record at Key Biscayne to 51-5, with one loss since 2001 — to her sister.