Miami - Serena Williams overcame a big deficit on Monday in her fourth-round match at the Sony Open to beat Dominika Cibulkova 2-6, 6-4, 6-2.
The top-ranked Williams, seeking her sixth Key Biscayne title and her first since 2008, was down a service break trailing 4-1 in the second set before she won five straight games. She swept the final three games and served out the match at love with three aces.
Williams's opponent in the quarter-finals will be No 5-seeded Li Na, who beat wildcard Garbine Muguruza 7-6 (8/6), 6-2.
Williams was flat-footed and impassive for much of her match against the No 13-seeded Cibulkova, but screamed encouragement during her comeback. She won despite 52 unforced errors, including 27 on her backhand.
In the half-empty stands, Williams fell behind in the sleepy atmosphere and one spectator shouted "Bor-ing!" early in the second set.
The 5ft3 inch Cibulkova kept Williams moving with shots into the corners and pounced on short shots to put away winners.
During changeovers, disco, salsa and Coldplay blared from the public address system. Williams eventually got going, her shots landing consistently just inside the lines. She won 10 consecutive points to begin her rally.
After sweeping the final five games of the second set, she went ahead to stay by breaking serve at love for a 2-1 lead in the final set. After closing out the victory with a 116-mph ace, she swung her fist in satisfaction.
Williams lost serve five times, hit 14 aces and had a 42-12 advantage in winners.
Results on Monday from the seventh day of the $8.5m WTA and ATP Masters Miami hardcourt tournament (x denotes seeding):
Andreas Seppi (ITA x16) bt Thomaz Belluci (BRA) 7-5, 4-6, 6-2
Andy Murray (GBR x2) bt Grigor Dimitrov (BUL x29) 7-6 (7/3), 6-3
Nicolas Almagro (ESP x10) bt David Goffin (BEL) 6-3, 6-4
Richard Gasquet (FRA x8) bt Mikhail Youzhny (RUS x28) 6-3, 6-4
Jo-Wilfried Tsonga (FRA x6) bt Jarkko Nieminen (FIN) 6-3, 6-3
Sam Querrey (USA x17) bt Milos Raonic (CAN x14) walkover
Marin Cilic (CRO x9) bt John Isner (USA x20) 6-3, 7-6 (7/3)
Tomas Berdych (CZE x4) bt Alejandro Falla (COL) 2-6, 7-6 (8/6), 6-4
Agnieszka Radwanska (POL x4) bt Sloane Stephens (USA x16) 4-6, 6-2, 6-0
Serena Williams (USA x1) bt Dominika Cibulkova (SVK x13) 2-6, 6-4, 6-2
Kirsten Flipkens (BEL x30) bt Ajla Tomljanovic (CRO) 6-7 (6/8), 6-4, 6-3
Li Na (CHN x5) bt Garbine Muguruza (ESP) 7-6 (8/6), 6-2
Roberta Vinci (ITA x15) bt Alize Cornet (FRA x32) 2-6, 6-4, 6-4
Jelena Jankovic (SRB x22) bt Sorana Cirstea (ROM x28) 6-0, 6-4
Sara Errani (ITA x8) bt Ana Ivanovic (SRB x12) 1-6, 6-4, 6-2
Maria Sharapova (RUS x3) bt Klara Zakopalova (21) 6-2, 6-2