Miami - Reigning US Open champs Andy Murray and Serena Williams stepped up their bids for another title while China's Li Na halted the giant-killing run of a Spanish teen to advance at the Miami Masters on Monday.
Second seed Murray blasted nine aces en route to a 7-6 (7/3), 6-3 victory over disgruntled Grigor Dimitrov to reach the fourth round of the hardcourt tournament.
Murray, who won this event four years ago, needed one hour and 54 minutes to oust the 29th-seeded Bulgarian Dimitrov.
Murray said he started slowly but soon figured out how to cause problems for his opponent.
"I was leaving the ball a bit short. Once I started to improve my depth a bit I made it tough for him and he started to make more mistakes after that," Murray said.
Dimitrov, 21, lost his composure when serving for the first set at 5-3, double-faulting three times to allow Murray to break back and the Scot duly prevailed in the tie-break.
It is the second time in as many tournaments that Dimitrov has cracked when serving for a set against a top-ranked player, serving up four double faults when he led world No 1 Djokovic 5-3 in the third round at Indian Wells.
"He probably got a little bit nervous, and that helped me," Murray said.
Murray, who committed 22 unforced errors to Dimitrov's 38, will face Italian Andrea Seppi in the fourth round, the 16th seed having beaten experienced Brazilian Thomaz Bellucci 7-5, 4-6, 6-2.
Meanwhile, eighth-seeded Frenchman Richard Gasquet rolled over 28th-seeded Russian Mikhail Youzhny 6-3, 6-4 and 10th-seeded Spaniard Nicolas Almagro beat Belgian David Goffin 6-3, 6-4. Gasquet faces Almagro in the fourth round.
Sixth seed Jo-Wilfried Tsonga, of France, cruised past Finland's Jarkko Nieminen 6-3, 6-3 as Canada's Milos Raonic withdrew from the tournament with strep throat just moments before his match against Sam Querrey.
In the women's draw, China's Li set up a quarter-final against world number one Williams with an error-strewn 7-6 (8/6), 6-2 win over teen Garbine Muguruza.
The 31-year-old Li is playing in her first tournament since suffering an ankle injury in January's Australian Open final.
Williams also struggled having to come from a set down to win 2-6, 6-4, 6-2 against Slovakia's 13th seed Dominika Cibulkova.
Williams, who is bidding for a record sixth Miami title, admitted she had not been at her best and at 4-1 down to Cibulkova in the second set and staring defeat in the face she had had to pull herself together.
"I was struggling mentally, I was making so many errors," said Williams, who regained the number one spot in Doha in February but has not played since.
"I could not pull myself together. Someone (a fan) kept telling me to relax and I listened to him."
Li - the 2011 French Open champion and two-time Australian Open finalist - made 39 unforced errors, including four double faults, and allowed her 19-year-old opponent to battle back to 5-5 in the first set having led 5-1.
Even at 6-1 in the tiebreaker Li faltered as Muguruza fought back to 6-6.
Li pulled herself together to take the next two points and the set, before wrapping up victory in the second set.
In the night match, third seed Maria Sharapova routed Czech Klara Zakopalova in straight sets 6-2, 6-2.
Sharapova, who is one of the favourites, has never won in Miami.
"It would be special to win this and add it to my collection," she said.
Sharapova advances to the quarter-finals where she will face eighth-seeded Sara Errani who rallied to beat former world No. 1 Ana Ivanovic 1-6, 6-4, 6-2.
Also, defending champ Agnieszka Radwanska defeated Sloane Stephens 4-6, 6-2, 6-0 to book a quarter-final clash with Belgian Kirsten Flipkens.
Results on Monday from the seventh day of the $8.5 million WTA and ATP Masters Miami hardcourt tournament (x denotes seeding):Men
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