Andy Murray's preparations for the US Open got off to a poor start when the Briton suffered a stunning 6-4 4-6 7-6 loss to unseeded Russian Teymuraz Gabashvili in the second round of the Citi Open in Washington.
The top-seeded Murray was never allowed to settle by the hard-serving Gabashvili, who blasted 11 aces and broke late in the third set before holding his nerve in the tiebreaker for his finest victory of the season.
The world number 53's previous high point of the campaign was his advance to the fourth round of the French Open but his victory over third-ranked Murray marks the fourth time the 30-year-old has secured a win against a top-10 opponent.
"It's obviously a disappointing match to lose but it wasn't like I was blown off the court in a match that I had no chance of winning. I feel like I put myself in a position to win the match," Murray said.
"It was the first hard-court match for four-and-a-half months with only four or five days of preparation, so I was happy with how I moved and did certain things on court.
"But there are some things for me to improve and hopefully with a few more days and having that match under my belt, I'll be able to make some improvements for the next few weeks."
Murray, who was awarded a first round bye, fell to Roger Federer in the Wimbledon semi-finals last month but had since helped Great Britain advance to the last four of the Davis Cup for the first time since 1981.
The US Open starts on August 31 in New York where Croatia's Marin Cilic will defend his title.