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 Monday.

Second seed Murray blasted nine aces en route to a 7-6 (7/3), 6-3 victory over disgruntled GrigorDimitrov 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 mother mistakes after that," Murray said. Dimitrov, last year's runner-up to Novak Djokovic, would have faced a tougher task had not the 21-year-old lost his composure when serving for the first set at 5-3. Dimitrov double-faulted 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 number one 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 ThomazBellucci 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 JarkkoNieminen 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 GarbineMuguruza.

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 DominikaCibulkova. 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 KlaraZakopalova 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 AgnieszkaRadwanska defeated Sloane Stephens 4-6, 6-2, 6-0 to book a quarter final clash with Belgian Kirsten Flipkens.