Britain's Emma Raducanu lost in straight sets in her first match since her thrilling Wimbledon debut last month.
The 18-year-old wildcard lost 6-3 6-2 to China's Zhang Shuai at the Mubadala Silicon Valley Classic.
Her return to the WTA Tour in San Jose followed her run to the fourth round at Wimbledon, which ended in retirement.
"She's a really nice girl and plays really, really great tennis," said world number 51 Zhang.
Zhang, a Grand Slam doubles champion at the Australian Open in 2019, raced into a 3-0 lead in the first set and was quickly 4-0 up in the second in the night match at the hardcourt tournament.