Dominic Thiem is through to the second round of the German Open in Hamburg after the top-seeded Austrian defeated Frenchman Corentin Moutet on Tuesday.
Thiem, the world number eight, won 6-4 6-2 in 82 minutes as he competed on clay for the first time since reaching the French Open final earlier this summer.
In the next round he will face Australian John Millman, who beat Germany's Jan-Lennard Struff 7-5 6-3.
Struff's compatriot Philipp Kohlschreiber, the fifth seed, crashed out as he was beaten 7-5 1-6 6-4 by Georgian Nikoloz Basilashvili, and the sixth seed also exited, Marco Cecchinato losing 6-4 3-6 6-4 to Gael Monfils.
Seventh seed Richard Gasquet won an all-French clash with Benoit Paire 7-5 6-3 and the eighth-seeded Fernando Verdasco got past Dusan Lajovic 6-3 4-6 7-6 (7-5).
At the Swiss Open in Gstaad, 2016 champion Feliciano Lopez, the eighth seed, advanced into round two as the Spaniard saw off Argentina's Federico Delbonis 6-3 7-6 (7-3).
Fifth seed Robin Haase - a twice-beaten finalist, including against Lopez two years ago - was defeated by Viktor Galovic, while sixth seed Joao Sousa lost to Facundo Bagnis.
At the Atlanta Open, five of the seven American players in action booked their places in the second round.
Qualifier Noah Rubin set up a last-16 meeting with second seed Nick Kyrgios after beating Australia's Thanasi Kokkinakis 6-3 6-4, while Taylor Fritz will face Hyeon Chung for a place in the quarter-finals after defeating Ramkumar Ramanathan 6-4 6-4.
Number eight seed Ryan Harrison rallied from a set down to defeat James Duckworth 4-6 7-6 (7/2) 6-1, Donald Young saved a match point to win 2-6 7-6 (7/5) 7-6 (8/6) against Ivo Karlovic, and Frances Tiafoe beat Marius Copil 6-4 6-4.
Meanwhile, British number three Cameron Norrie went through to the second round after beating Malek Jaziri 7-5 6-0.