Top-seeded Daniil Medvedev has recovered from a shaky start to begin his defence of the St Petersburg Open title with a 3-6 6-3 6-0 win over Richard Gasquet.
Medvedev hit 22 aces to secure his first win since losing to Dominic Thiem in the US Open semi-finals, having lost in the first round in Hamburg and at the French Open.
Medvedev set up a second-round match with Reilly Opelka, who earned a 3-6 7-6 (7-5) 6-2 win over qualifier Nino Serdarusic.
Stan Wawrinka booked his spot in the quarter-finals with a 6-1 3-6 6-3 win over Evgeny Donskoy, a wild-card entry who has now competed in the St Petersburg tournament 12 times without ever getting past the second round.
Wawrinka could next meet second-seeded Canadian Denis Shapovalov, who needed just 1 hour, 3 minutes to wrap up his 6-2 6-3 first-round win over lucky loser Viktor Troicki.
Shapovalov was coming off a second-round loss at the French Open. He takes on qualifier Ilya Ivashka in the second round for a quarter-final spot against Wawrinka.
Ugo Humbert will play third-seeded Andrey Rublev in the second round after Humbert defeated qualifier Pavel Kotov 4-6 6-2 6-3.
Cameron Norrie claimed his 50th ATP Tour victory and reached the last eight by beating Miomir Kecmanovic in three sets.
The Brit won 7-5 6-7 (7-5) 6-3 against the 21-year-old Serbian, who is 34 places higher than Norrie in the rankings.