Kvitova inspired by Serena to upset Muguruza at US Open
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The Czech 21st seed looked to be heading for elimination as she trailed 5-2 in the third after the opening two sets were shared.
But the two-time Wimbledon champion rallied superbly to level at 5-5 and then fought off two Muguruza match points at 5-6 before holding to force a tie break and sealing a 5-7, 6-3, 7-6 (12/10) victory at the Arthur Ashe Stadium.
Kvitova said she had drawn inspiration from Serena Williams' defeat on Friday night, when the 23-time Grand Slam champion saved five match points before eventually being eliminated.
"I watched Serena last night and it was amazing how she was saving match points, so I tried to do the same today," Kvitova said.
"It was a great match, a great fight. She was serving very well and I was dealing with nerves," the 32-year-old added.