The top-ranked Williams lost the first five games Wednesday but rallied 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 faces Agnieszka Radwanska in the last of the women's 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 the unseeded Li failed to convert all six of her break-point chances in the pivotal second set.
