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Gauff, who was a beaten finalist in Rome last year, edged closer to another shot at glory with a 5-7 6-0 6-4 victory in two hours and 10 minutes.

The No.3 seed started strongly with a break of serve in a mammoth second game, only for Sierra to hit back immediately.

Gauff's frustrations boiled over after she was broken at 5-5, with the American hitting her own head with her racquet.

She lashed out again when Sierra clinched the opening set.

However, the American responded emphatically, with a second-set bagel, to tee up a decider.

Yet she needed to dig deep after finding herself 3-0 down.

But she won six of the last seven games, setting up a clash against fellow Americans Taylor Townsend or Iva Jovic.