Heather Watson lost in the first round of Cincinnati's Western & Southern Open as China's Shuai Zhang punished an erratic display, winning 6-3 2-6 7-5.
Watson rose into the world's top 50 by reaching the Rogers Cup third round last week, but was inconsistent as she lost in just over two hours.
The British number one finished the match with a first-serve percentage of just 47%.
Watson's compatriots Andy Murray and James Ward are in the men's draw.
|Heather Watson's performances at the US Open|
|2013: First round - lost to Simona Halep 4-6 6-4 6-2|
|2012: First round - lost to Li Na 6-2 6-3|
|2011: First round - lost to Maria Sharapova 3-6 7-5 6-3|
Murray, afforded a bye to the second round, will play Portugal's Joao Sousa on Wednesday while Londoner Ward takes on Spain's Guillermo Garcia-Lopez on Tuesday.
Watson, 22, won her previous meeting with Zhang in Estoril in 2012 in straight sets, but left too many balls short for her opponent to capitalise on in the first set.
Coach Diego Veronelli issued a raft of instructions to Watson at the changeover, but it seemed his client was doomed to an early exit as she surrendered serve at the start of the second set, before rallying to take it 6-2.
In the final set Watson clawed the world number 31 back to 3-3 from 3-0 adrift and saved two match points at 5-4 down, but was eventually undone as Zhang advanced to a meeting with either former world number one Caroline Wozniacki or Slovakia's Magdalena Rybarikova.
Watson is in the main draw for the US Open which begins on 25 August. The Connecticut Open, the final warm-up event which begins on 17 August, could offer Watson a wild card.