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The reigning champion won 3-6 6-3 6-2 in two hours and 12 minutes to set up a last-four clash against either Iva Jovic or Anna Kalinskaya.

After the pair exchanged breaks in the opening set, Shnaider broke once again in the eighth game on her way to taking the first.

Pegula was stronger on serve in the second, conceding no break points, and she eventually converted her fifth break point opportunity to take the match to a deciding set.

Shnaider fought to a 2-0 lead in the third, but No.1 seed Pegula reeled off six consecutive games to clinch victory, which she did with her fourth match point.

Madison Keys also came from a set down to defeat No.3 seed Belinda Bencic 4-6 6-3 6-2 in their last eight match-up.

Keys will face either Yuliia Starodubtseva or McCartney Kessler in the semi-finals in Charleston.