Pegula into US Open quarter-finals
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The 28-year-old eighth seed shrugged off a rain delay on the Arthur Ashe Stadium court to win 6-3, 6-2 and ease into a last eight meeting with either world number one Iga Swiatek or Germany's Jule Niemeier.
Pegula, whose parents are the billionaire owners of the Buffalo Bills NFL franchise, was always in control against out-of-sorts Czech veteran Kvitova, who had taken down Spain's Garbine Muguruza in the third round.
Pegula scored a break to go 4-3 up in the first set following the rain delay and then held for a 5-3 lead before breaking again for 6-3.
Kvitova appeared to raise her game in the second set and broke immediately before holding for a 2-0 lead.
But Kvitova's game disintegrated thereafter – the Czech made 24 unforced errors in total – and Pegula reeled off six straight games to take the second set, clinching the match when Kvitova yanked a wild return wide.
Pegula has reached quarter-finals at the Australian Open and French Open this season but had never gone further than the third round in five previous appearances at the US Open.
She joins fellow American Coco Gauff in the last eight while compatriot Danielle Collins can join them when she faces Aryna Sabalenka later Monday.