American Reilly Opelka saved four break points in the final set before beating defending champion Daniil Medvedev 2-6, 5-7, 4-6 to reach the quarter-finals of the St Petersburg Open.
Medvedev has lost four of his last five matches, including the US Open semifinal against Dominic Thiem and he lost his his way in this match as Opelka broke service twice in the second set to clinch the match.
Opelka faces a quarterfinal against No. 7 seed Borna Coric, who beat wild-card Roman Safiullin 6-3, 7-5 after coming back from 3-1 down in the second set.
Denis Shapovalov had little trouble beating Belarusian qualifier Ilya Ivashka 6-1, 6-4 and Andrey Rublev came back from a set down to win 4-6, 6-4, 7-5 against Ugo Humbert. Brit Can Norrie is up next.
Karen Khachanov won an all-Russian match with Aslan Karatsev 4-6, 7-5, 6-3 and Milos Raonic beat Alexander Bublik 6-3, 6-2.