Defending champion Leonardo Mayer progressed to the second round of the German Open with a straight-sets victory over Spaniard Albert Ramos-Vinolas.
The unseeded Argentinian, who also won the tournament in 2014, secured a 6-3 6-2 success in Hamburg to set up a last-16 meeting with either sixth seed Marco Cecchinato or Frenchman Gael Monfils.
Fellow Argentinian Diego Schwartzman, seeded second, is also through to the next round, although he needed three sets to see off 19-year-old wildcard Casper Ruud.
The world number 12 recovered from a slow start to win 6-4 2-6 6-2 and will play Daniel Masur next after he defeated fellow German Maximilian Marterer 6-3 6-4.
David Ferrer, finalist in 2014, was knocked out, losing 7-5 5-7 6-3 to 17-year-old German wildcard Rudolf Molleker.
Molleker's reward for defeating the experienced Spaniard is a second-round meeting with Jozef Kovalik after he dumped out fourth seed Damir Dzumhur 6-2 6-4.
Last year's runner-up Florian Mayer also exited the tournament on Monday, failing to capitalise on a strong start before losing 2-6 6-1 7-6 (7/4) to third seed Pablo Carreno-Busta.
At the Swiss Open in Gstaad, seventh seed Guillermo Garcia-Lopez suffered a straight-sets defeat at the hands of fellow Spaniard Roberto Carballes Baena.
World number 67 Garcia-Lopez was broken in the opening game of the match but managed to take the first set to a tie-break, before struggling in the second set as he was beaten 7-6 (7/3) 6-2.
Carballes Baena will face Japan's Taro Daniel in the last 16 after he edged past Swede Elias Ymer 7-6 (11/9) 7-5.
Estonian Jurgen Zopp came from a set down to beat fellow qualifier Yannick Hanfmann 3-6 6-3 7-6 (7/4) and set up a second-round meeting with top seed Fabio Fognini, while Canadian wildcard Felix Auger-Aliassime beat Guido Andreozzi 6-3 7-6 (7/3).
In Monday's other match, Spain's Jaume Munar was a comfortable 6-1 6-0 winner against compatriot Adrian Menendez-Maceiras and will play second seed Roberto Bautista Agut next.
At the Atlanta Open, Australian teenager Alex de Minaur battled back to set up a last-16 meeting with top seed John Isner.
The 19-year-old dropped the opening set against Polish lucky loser Hubert Hurkacz, before winning successive tie-breaks to progress 1-6 7-6 (7/0) 7-6 (7/5).
Slovakian Lukas Lacko is also through to the next round after he comfortably beat qualifier Prajnesh Gunneswaran 6-2 6-2.