Following a first-round bye, 2023 US Open champion Gauff was sent packing on Tuesday, beaten 6-4 7-5 in the second round by the world No.54.
Gauff hadn't fallen to an opponent ranked outside the top 50 since a loss to Sofia Kenin (128th) at Wimbledon in 2023.Â
It was a first top-10 win for 25-year-old Kessler.
She will now face 14th seed Karolina Muchova, who ousted 2021 US Open champion Emma Raducanu 7-6 (8-6) 6-4.
Raducanu, who appeared to be in tears as she went to the umpire's chair after the second game, fell behind 4-0 before forcing a tiebreaker.
Raducanu was broken again in the first game of the second set. After failing to convert any of the three break points she earned in the sixth game, it was Muchova who prevailed.
Earlier, second-seeded Iga Swiatek beat Victoria Azarenka 6-0 6-2 and will next face Dayana Yastremska for a spot int the quarter-finals.
Top-seeded Aryna Sabalenka eliminated Veronia Kudermetova 6-3 6-4, and defending champion Jasmine Paolini defeated Eva Lys 6-2 7-5.
Elena Rybakina , Jessica Pegula and Paula Badosa also advanced.
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