James Blake said a slippery court had affected his game after he lost to Philipp Kohlschreiber in the Gerry Weber Open quarter-finals in Halle.
Organisers closed the roof after a brief shower in the second set but Blake, who broke two vertebrate on court in 2002, feared for his safety.
"After the rain started, Philipp slipped and I slipped twice," he said.
Despite his concerns, the American third seed had already been outplayed by the German, who won 6-4 6-3.
Blake conceded after the match: "Philipp played very well and some calls didn't go my way.
"It happens like that sometimes, but these things just tend to snowball when they go against you.
"I just didn't want to twist my knee this close to Wimbledon."
Kohlschreiber, who won on clay in Munich last month, faces eighth seed Marcos Baghdatis in Saturday's semi-finals.
The Cypriot player saw off German number three Florian Mayer 6-3 6-3.
"My confidence was high at the start of the season, but it's just getting higher and higher now," said the 23-year-old.
Frenchman Marc Gicquel made a painful exit from Halle after he was forced to retire from his quarter-final against Jarkko Nieminen.
Gicquel was suffering the after-effects of being hit in the groin by Benjamin Becker's 208kph serve on Thursday and retired after Nieminen won the first set 6-4.
"I didn't have much energy and after the first set, I just knew I didn't have enough left in me to go on," said Gicquel.
"I spent most of Thursday night not feeling well and I was sick a few times."
Finland's Nieminen faces Tomas Berdych in the other semi-final after the Czech fourth seed advanced when Mikhail Youzhny pulled out with a back injury.
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