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.
Elsewhere, Croatian top seed and 2012 Queen's champion Marin Cilic progressed with a 6-3 6-4 win over Spain's Fernando Verdasco.
Luxembourg's Gilles Muller won 7-6 (9-7) 7-6 (8-6) against Canadian teenager Denis Shapovalov.
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.
On his way to the last four he earned the biggest win of his career against American world number 10 John Isner, then pushed French number one Lucas Pouille in their French Open second-round match.
But going into Monday's match against Wawrinka, Norrie had only won two tour-level matches on grass.
Wawrinka, himself, does not have a good record on the surface in recent years, winning only one match on grass in the past three years.
However, his greater pedigree - and power - told as Norrie was far from his best.
Norrie saved a break point in his first service game, only for the powerful Wawrinka to take another opportunity for a 3-1 lead.
Wawrinka dominated the first set, based on some big serving and dominant groundstrokes, and broke again in the final game to wrap up the opener in just 27 minutes.
And the former Australian Open, French Open and US Open champion broke again early in the second set, going on to seal a comfortable win in less than an hour.
Wawrinka sat out the second half of the 2017 season after needing knee surgery following his early Wimbledon exit last year, before suffering another setback earlier this year.
Victory over Norrie was only his fifth since his first-round SW19 defeat by Daniil Medvedev in July.
"I'm really happy, it was a really good start - is never easy the first match on grass. I served well and I'm happy to be back healthy and fit," said Wawrinka, who lost in the first round of the French Open last month.
"In general it has been improving and now I feel really good on the court, I feel strong physically.
"I know I will have some ups and downs. I need to be patient, especially mentally, and ready to accept the frustration of not playing my best tennis."