Miami - World No 1 Serena Williams advanced to the semi-finals of the $8.5 million WTA and ATP Masters Miami hardcourt event on Tuesday by defeating Chinese fifth seed Li Na 6-3, 7-6 (7/5).
Breaking Li three times in each set, Williams surrendered her own serve three times in the match to win after one hour and 50 minutes. She had three aces and six double faults to one ace and seven double faults for Li.
Williams, trying to become the first six-time winner in Miami history, will face either defending champion and fourth seed Agnieszka Radwanska of Poland or 30th-seeded Belgian Kirsten Flipkens in the semi-finals.
Williams won Miami titles 2002, 2003, 2004, 2007 and 2008. The 15-time Grand Slam champion also captured last year's US Open, Wimbledon and Olympic crowns.
Li made a strong return to the tour at Miami after an ankle injury suffered in losing the Australian Open final.
Williams improved to 7-1 against Li with her sixth victory in a row, Li's only win coming at Stuttgart in 2008.
In the men's draw Spanish third seed David Ferrer advanced to the quarter-finals with a 6-4, 6-2 victory over Japan's 13th-seeded Kei Nishikori.
Ferrer won 84 percent of his first-serve points and 5-of-9 break chances to eliminate Nishikori and book a last eight-date against Austria's Jurgen Melzer, who defeated Spain's Albert Ramos 2-6, 6-3, 6-3.
Fifth-ranked Ferrer improved to 3-2 in his meetings with Nishikori, having avenged a round-of-16 loss at the Olympics by defeating the Japanese player in the fourth round of this year's Australian Open.
World No. 1 Novak Djokovic of Serbia meets German 15th seed Tommy Haas in a later fourth-round match and British second seed Andy Murray will meet Italian 16th seed Andreas Seppi.
Murray lost to Djokovic in last year's Miami final.
Latest results on Tuesday from the eighth day of the $8.5m WTA and ATP Masters Miami hardcourt tournament (x denotes seeding):Men
David Ferrer (ESP x3) bt Kei Nishikori (JPN x13) 6-4, 6-2
Jurgen Melzer (AUT) bt Albert Ramos (ESP) 2-6, 6-3, 6-3
Richard Gasquet (FRA x8) bt Nicolas Almagro (ESP x10) 6-7 (3/7), 7-5, 7-6 (7/3)
Andy Murray (GBR x2) bt Andreas Seppi (ITA x16) 6-2, 6-4
Tomas Berdych (CZE x4) bt Sam Querrey (USA x17) 6-1, 6-1
Marin Cilic (CRO x9) bt Jo-Wilfried Tsonga (FRA x6) 7-5, 7-6 (7/4)
Gilles Simon (FRA x11) bt Janko Tipsarevic (SRB x7) 5-7, 6-2, 6-2
Tommy Haas (GER x15) bt Novak Djokovic (SRB x1) 6-2, 6-4
Serena Williams (USA x1) bt Li Na (CHN x5) 6-3, 7-6 (7/5)
Agnieszka Radwanska (POL x4) bt Kirsten Flipkens (BEL x30) 4-6, 6-4, 6-2