Cameron Norrie fell to an early exit at the Monte Carlo Masters as he lost to Francisco Cerundolo in the first round.
The British number one, ranked 13th in the world, was beaten 6-3 6-4 by Argentina's Cerundolo on the Mediterranean clay.
Fellow Briton Jack Draper returned from injury with a 6-3 7-5 win in his opening tie against another Argentine, Sebastian Baez.
The 21-year-old faces 10th seed Hubert Hurkacz of Poland in the second round.
Draper was forced to retire during a fourth-round match with world number two Carlos Alcaraz in Indian Wells last month because of an abdominal problem.