Britain’s Kyle Edmund lost in the first round of the Bett1 Hulks Indoors event in Cologne, Germany.
Edmund, 25, ranked 47th in the world, was beaten 7-5 7-6 (7-1) by South Africa’s Lloyd Harris, the world number 92.
Andy Murray is in first-round action later on Tuesday.
The 33-year-old three-time Grand Slam champion faces Spain's Fernando Verdasco, 36, with the winner to face world number seven Alexander Zverev.
Zverev, 23, who reached the final of the US Open last month, is the tournament's top seed and was given a bye for round one.
For Edmund, who won the first set against Novak Djokovic in the US Open second round before losing in four sets, it is his second successive first-round exit after losing to world number 113 Marco Cecchinato at the ATP Masters in Rome in September.