Linda Nosková surprised Jessica Pegula to win the Berlin Tennis Open grass event 6-4, 4-6, 6-3 on Sunday following a severe thunderstorm and a big delay.
Lightning, high winds and torrential rain led to the Berlin venue being evacuated before the final, which eventually started five hours late.
Organizers said they were prepared for the forecast weather and took precautionary measures but that "the intensity and speed of the storm made additional actions necessary at very short notice."
The deluged ground took time to drain and infrastructure needed fixing after high winds at the Steffi Graf Stadium. A tunnel leading to a nearby railway station was also flooded.
But Czech eighth seed Nosková initially coped with the chaos better and raced to a 3-0 lead before wrapping up the first set.
US third seed Pegula, who had beaten world number one Aryna Sabalenka in Saturday's last four, hit back in the second set but then slipped 2-0 behind in the decider.
Nosková did not look back as she sealed her second WTA title.
"It is very incredible and unforgettable for me," she said. "You never forget the first one but you always never forget the second one as well."
The Czech was due to finish her doubles semi-final afterwards, but it appeared to be scrapped.
The Berlin grass event was a warm-up for the Wimbledon Grand Slam tournament, which starts a week on Monday.
