German Philipp Kohlschreiber stunned second seed Marin Cilic 3-6, 6-3, 6-4 on Friday to move into the semi-finals of the Rotterdam World Tennis at the eighth attempt. (Read more tennis stories here)
The 32-year-old improved to 6-3 over former US Open winner Cilic as he won a comeback duel in just over two hours.
"I gained a lot of confidence after my break in the second set and it gave me belief that I could win the match", Kohlschreiber said.
"It feels great to stun a great player like Marin in such a big tournament.
"Sometimes everything falls into place. Looking back, I made the right decisions on the decisive moments."
Kohlschreiber, ranked 33rd in the world, fought off 15 Cilic aces while producing nine of his own, along with 38 winners and 20 unforced errors.
Fifth seed Gael Monfils stopped an upset bid from German teenager Alexander Zverev, 7-6 (7/4), 6-3, with the Frenchman making life hard on himself.
Monfils was broken while serving for the first set before salvaging it in a tiebreaker.
Leading 3-1 in the second, he sent over a double fault to suddenly face three break points, saving them all for 4-1
The showman finally finished off the win on a second match point after saving three more break points in the final game.
He will next face Kohlschreiber against who he boasts a 11-2 career lead.
Slovakia's Martin Klizan saved five match points to stun in-form Spanish sixth seed Roberto Bautista Agut 6-7 (5/7), 7-6 (8/6), 6-0.
Klizan was unaware of his impressive rearguard action against Bautista Agut, the winner of the Sofia title last week and who was bidding for a 15th match win of the season.
"I thought it was only two match points," Klizan said. "All I could do out there was to fight on every point.
"There was not really a turning point in the match, every ball was difficult, there were no easy points. We had a lot of rallies and it was good to watch."
Bautista Agut, ranked 17th, missed a chance for a straight-sets victory when he was broken for 5-5 in the second set despite saving four break points in the game.
Klizan seized his chance, holding for 6-5 and won a tiebreaker to take the match into a deciding third set, where he dominated to love with the win in two and three-quarter hours.
Klizan will next face French qualifier Nicolas Mahut, who put out eighth seeded Viktor Troicki of Serbia, 6-2, 6-1.
Mahut, ranked 54th in the world, has a winning past in the Netherlands, earning a pair of trophies at the grass event at Rosmalen in 2013 and 2015.