Tennis - Serena roars back to win in Miami

Serena Williams had to come back from a set down to beat Dominika Cibulkova 2-6 6-4 6-2 in the fourth round of WTA Miami.

The world number one is seeking her sixth title in Key Biscayne, but struggled for rhythm for more than an hour as Cibulkova threatened an upset in front of a half-full stadium.

Indeed Cibulkova had taken the first set and led 4-1 in the second before Williams rallied, finishing off the contest with three aces. It was in stark contrast to her earlier display, and she racked up 52 unforced errors in the course of the match.

''I was just struggling,'' Williams said. ''I was making so many errors and couldn't pull myself together.

''I saw my dad on the sideline, and he was so calm and so relaxed, and I was just like, 'OK, Serena, you can do this.' I took that energy and started playing better.''

Williams will face Li Na in the quarter-finals after the Chinese beat wild card Garbine Muguruza 7-6 6-2.

Maria Sharapova powered her way past another opponent in Miami as the Russian world number two remains on target to finally claim a tournament that has brought her plenty of heartbreak.

The third seed shrugged off the challenge of Klara Zakopalova, easing to a 6-2 6-2 victory over the Czech to advance to the quarter-finals and inch closer making it fifth time lucky in Florida.

Sharapova was beaten in the last two finals after also falling in the tournament decider in 2005 and 2006, and her determination to go one better this year was evident against Zakopalova, who was helpless in the face of the Russian's power.

Sharapova now meets eighth seeded Italian Sara Errani, who came back from a disastrous opening set to clinch a 1-6 6-4 6-2 over Ana Ivanovic of Serbia.

Errani will be joined in the quarter-finals by compatriot Roberta Vinci, who also had to come from behind to down Frenchwoman Alize Cornet 2-6 6-4 6-4.

Awaiting Vinci in the last eight is Jelena Jankovic, who downed Sorana Cirstea in straight sets.

Another top seed who struggled but eventually progressed was Agnieszka Radwanska, who needed three sets to overcome American rising star Sloane Stephens.

The 20-year-old, a semi-finalist at the Australian Open, took the first set 6-4 before fourth-seeded Radwanska bounced back, dropping just two more games to win 4-6 6-2 6-0.

Her reward is a quarter-final against Kirsten Flipkens, who ousted Ajla Tomljanovic in three tight sets.

Monday's fourth-round results

3-Maria Sharapova (Russia) beat 21-Klara Zakopalova (Czech Republic) 6-2 6-2

22-Jelena Jankovic (Serbia) beat 28-Sorana Cirstea (Romania) 6-0 6-4

8-Sara Errani (Italy) beat 12-Ana Ivanovic (Serbia) 1-6 6-4 6-2

15-Roberta Vinci (Italy) beat 32-Alize Cornet (France) 2-6 6-4 6-4

4-Agnieszka Radwanska (Poland) beat 16-Sloane Stephens (US) 4-6 6-2 6-0

1-Serena Williams (US) beat 13-Dominika Cibulkova (Slovakia) 2-6 6-4 6-2

30-Kirsten Flipkens (Belgium) beat Ajla Tomljanovic (Croatia) 6-7(6) 6-4 6-3

5-Li Na (China) beat Garbine Muguruza Blanco (Spain) 7-6(6) 6-2

15-Roberta Vinci (Italy) beat 32-Alize Cornet (France) 2-6 6-4 6-4