Top-seed Rafael Nadal and Gael Monfils both struggled on their least-favourite surface before setting up a semi-final at the first edition of the Stuttgart Open to be played on grass.
The former July clay-court tournament is now part of the three-week run-up to Wimbledon after being approached three years ago to consider making the change of surface and date.
My game was at a good level, I'm happy with how I handled things
Nadal needed almost two and a half hours to defeat fifth-seed Bernard Tomic 6-4, 6-7, 6-3, beating the Australian for the third time as they played on grass for the first time.
Despite his hard slog, Nadal is confident with his game ahead of Wimbledon and after a poor clay season where he was dethroned as French Open champion.
"I'm feeling strong and I'm motivated, I hope to have a good second half of the season. I played well today. It was a good match for me," said Nadal.
"I had a great first set and in the second he played well. My game was at a good level, I'm happy with how I handled things. It was a very positive win, a good one for me."
Meanwhile, Monfils fired 17 aces in a 7-5, 3-6, 6-3 quarter-final victory over Philipp Kohlschreiber.
Monfils struck for his 10th win over Kohlschreiber against just two losses - the German won their grass-court semi-final in Halle in 2011 and also put out Monfils on the Stuttgart clay in 2013.
The second semi-final will be played between US Open winner Marin Cilic, who beat qualifier Mischa Zverev 4-6, 7-6, 7-6 and eighth-seed Viktor Troicki, a winner over Australian Sam Groth 7-6, 6-1.