British No 4 Aljaz Bedene was beaten by Denis Istomin in the first round of the World Tennis Tournament in Rotterdam on Monday.
Bedene was broken once in the opening set, before squandering a 5-3 lead in the second to eventually lose on a tie-break 6-3 7-6 (7-3).
The Slovenia-born player had beaten Istomin 24 hours earlier in straight sets in the second round of qualifying.
Elsewhere, Tomas Berdych needed a break in the final game to seal a 7-6 (7-3) 6-4 win over Romanian qualifier Marius Copil.
Fourth seed Berdych, winner of the indoor title at the Ahoy Stadium three years ago, saved all three break points he faced while breaking once to reach the second round.
Frenchman Pierre-Hugues Herbert booked the first of the second-round spots, beating Spanish veteran Feliciano Lopez 7-6 (7-5), 7-6 (7-5).
Dutchman Robin Haase defeated German Florian Mayer 7-5 7-6 (7-3) while Evgeny Donskoy of Russia dispatched Marcel Granollers 6-4 6-4.
Former US Open winner and top seed Marin Cilic is set to begin his campaign against Frenchman Benoit Paire on Tuesday, live on Sky Sports.