British number two Cameron Norrie was knocked out of the Fever-Tree Championships at Queen's Club in straight sets by three-time Grand Slam champion Stan Wawrinka.
Wawrinka, 32, won 6-2 6-3 against the world number 80.
The Swiss, who has dropped to 261 in the world after injury, meets either British teenager Jay Clarke or American San Querrey in the second round.
Andy Murray makes his return to competitive action on Tuesday.
Earlier, Luxembourg's Gilles Muller won 7-6 (9-7) 7-6 (8-6) against Canadian teenager Denis Shapovalov.
Wawrinka too strong for Norrie
Norrie has enjoyed a rapid rise this year, climbing into the top 100 for the first time and reaching his first ATP Tour semi-final on the clay in Lyon.