Rafael Nadal defeated Lukas Lacko 7-5, 6-1 at the Gerry Weber Open on Thursday in the Spaniard's first singles since clinching his record seventh French Open title.
Nadal twice broke the Slovak in a first set that lasted 50 minutes, before finally making his superiority count against Lacko in a second set that was half as long.
"In the second set I think I did everything better," said Nadal, who defeated top-ranked Novak Djokovic at Roland Garros on Monday for his 11th Gram Slam title.
"Every hour on court works well for me - on grass - because I don't have a lot of time to practice. I'm happy to be through, that's the main thing."
Nadal will next play defending champion Philipp Kohlschreiber, who rallied from a set down and struck 11 aces to defeat Lukasz Kubot of Poland 6-7 (5), 6-1, 6-3 in just over two hours.
"I was too anxious and missed too many chances," Kohlschreiber said of the first set in which he failed to take two break points.
"(But) the early break to make it 2-0 (in the second set) was very good for me."
The 33rd-ranked Kohlschreiber has never beaten Nadal in eight attempts.
Also, wild card Tommy Haas of Germany, the 2009 winner, eliminated sixth-seeded Marcel Granollers of Spain 6-3, 6-4 for a quarterfinal date with third-seeded Tomas Berdych, the 2007 winner.
Haas, the oldest player in the singles draw at 34, broke Granollers once in the first set before claiming a decisive break in the ninth game of the second.
Five-time winner Roger Federer was playing late against Germany's Florian Mayer.