Li Na of China hits a return during her victory over Garbine Muguruza of Spain at the Sony Open tennis tournament in Key Biscayne, Florida, March 25, 2013. [Photo/Agencies]

Garbine Muguruza of Spain hits a backhand during her loss to Li Na of China at the Sony Open tennis tournament in Key Biscayne, Florida, March 25, 2013. [Photo/Agencies]

NEW YORK - China's Li Na reached the quarterfinals of the Miami Masters on Monday by beating Spanish teenager Garbine Muguruza 7-6(6), 6-2.

The 31-year-old Li, who is playing in her first tournament since suffering an ankle injury in January's Australian Open final, will face world number one Serena Williams in the last eight.

Williams also struggled in her match, coming from one set down to win 2-6, 6-4, 6-2 against Slovakia's 13th seed Dominika Cibulkova.

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 tiebreak Li faltered as 73rd-ranked Muguruza fought back to 6-6.

Li was more clinical in the second set breaking Muguruza two times for the victory.