China's Li Na reached the quarter-finals of the Miami Masters on Monday with an error strewn 7-6 (8/6), 6-2 win over giantkilling Spanish teenager Garbine Muguruza.
Li - who is playing in her first tournament since suffering an ankle injury in January's Australian Open - 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 tiebreker Li faltered as 73rd-ranked Muguruza, who had claimed the scalps of the seeded Caroline Wozniacki and Anastasia Pavlyuchenkova in previous rounds, fought back to 6-6.
However, fifth-seeded Li pulled herself together to take the next two points and the set.
Li was more clinical in the second set breaking Muguruza, who was born in Venezuela but has a Spanish father who now lives in Barcelona, in the second game and then again when closing out the tie as her opponent served a double fault on match point.
Li will play top seed Serena Williams in the quarter-finals after the American came back 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 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. I saw my dad courtside and he was so calm, it helped me."