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 world number 51 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.
"I am at the beginning of my journey so every opportunity I get I'm grateful as I'm constantly learning," she said.
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.
Raducanu's serve was broken four times in all and while she had two break points against her experienced opponent early in set two, she failed to capitalise.
"On the WTA Tour every match is going to be difficult and there aren't going to be many good matches," Raducanu said.
"It is my second WTA Tour tournament. What I learned is how to try and deal with someone when they are playing very well and you feel like you are not doing much wrong but you are constantly on the back foot.
"Stepping up to this level, I feel she just dictated me. If I keep going this way at some point it will pay off."
Zhang hit 29 winners in sealing a comfortable victory in 82 minutes.
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