Jan-Lennard Struff spoiled Denis Shapovalov's 20th birthday by posting a 5-7, 6-3, 6-1 victory on Monday in the first round of the Monte Carlo Masters at Monaco.
The 28-year-old German bounced back from losing the opening set to easily win the final two while prevailing over the 15th-seeded Canadian in one hour, 58 minutes on the clay-court surface.
Struff posted four aces and won 71.7 percent of his first-serve points.
Shapovalov was plagued by 51 unforced errors against just 22 winners.
Struff next faces Bulgaria's Grigor Dimitrov.
Two other seeded players lost on Monday: No. 12 Nikoloz Basilashvili of Georgia and No. 17 Brit Kyle Edmund.
Hungary's Marton Fucsovics scored six aces to record a 7-5, 3-6, 6-1 win over Basilashvili in two hours, three minutes.
Argentina's Diego Schwartzman rallied to register a 4-6, 6-3, 6-1 win over Edmund. Schwartzman was down 0-3 in the second set before recovering to win 12 of the final 13 games in the two-hour, two-minute match.
The highest-seeded winner on Monday was No. 10 Daniil Medvedev, who needed just 55 minutes to trounce Portugal's Joao Sousa 6-1, 6-1.
The Russian had five aces while posting his 18th victory of the season, tying Roger Federer for most on the Tour.
"It was a good match. Of course, it is not easy to start the first match on clay," Medvedev said after his victory. "You want to see how your practices have been going (and if) they worked out. In this moment I am happy with my performance but looking forward to the next matches because it is only the start."
No. 11 seed Marco Cecchinato of Italy won the first four games of his match with Damir Dzumhur before the Bosnian retired with an abdominal injury. Cecchinato will face Switzerland's Stan Wawrinka in the second round.
No. 13 seed Fabio Fognini of Italy rallied to outlast Russian qualifier Andrey Rublev 4-6, 7-5, 6-4. Fognini faced a 1-4 deficit in the second set before storming back to win in two hours, 30 minutes.
Also advancing was No. 16 seed David Goffin, who cruised to a 6-4, 6-1 victory over Argentina's Guido Andreozzi. Goffin had five aces while winning in 85 minutes.
In a match between non-seeded players, Germany's Philipp Kohlschreiber cruised to a 6-1, 6-3 win over Japan's Taro Daniel in 67 minutes.
Kohlschreiber next faces top-seeded Novak Djokovic. Kohlschreiber stunningly upset Djokovic in straight sets during last month's BNP Paribas Open at Indian Wells.
--Field Level Media