"She said: 'you're going to win this in the future," the 23-year-old Spaniard recalled last month during a TV interview.
"But I don't think she meant (while) playing her sister."
Little did Serena know that Muguruza would overtake her a year later for the French Open title and then make good on her prophecy by besting sister Venus at Wimbledon this year.
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Now with Serena sidelined for the impending birth of her first child, the two-times grand slam winner is delivering on the promise that could see her become the new face of women's tennis.
With her devastating 6-1 6-0 romp of Simona Halep to claim the Cincinnati Open on Sunday, Muguruza rose to third in the world rankings.
And with two wins in her last four tournaments, she arrives at Flushing Meadows in New York with a seemingly good formula to reach the second week at the U.S. Open.
"Playing three tournaments (since Wimbledon) and lifting a trophy is a great preparation and I hope it's going to help me," Muguruza told reporters.
"But I know even if you play a terrible match before a grand slam it doesn’t really matter, or if you win a tournament. We’ll see how it goes.”
For all of Muguruza's natural gifts, including her powerful groundstrokes, wingspan and court coverage, she has yet to realise her best in New York and in four attempts has never advanced past the second round of the U.S. Open.
Muguruza has admitted a natural affinity for French Open clay and a reverence for the grass courts of Wimbledon, but the hard courts of Flushing Meadows have been a puzzle.
"I feel like I'm going to try and not have a lot of expectations with New York," said Muguruza. "Every time I have a little bit, it holds me a little bit back. I'm just going to go out there from zero. Forget about what happened previous years. It's a new tournament."
(Reporting by Jahmal Corner in Los Angeles; Editing by Frank Pingue)
