Tsitsipas was unable to build on his momentum from his shock win over Rafael Nadal
Andy Murray said he “lost respect” for Stefanos Tsitsipas after the teenager ended his great summer with defeat by the world number 39 at the Cincinnati Masters.
Murray was bidding to become the first British man since 1977 to reach three successive Masters titles since his breakthrough year of 2013.
But the 27-year-old Scot lost 6-3 7-6 (7-4) to Tsitsipas to end his tournament with a third defeat in four matches.
“When you lose a match like that, it’s a tough pill to swallow,” he said.
“You do feel sorry for yourself, especially going into the US Open with I know how much work I’ve put in.
“When you have a big match like that, all those things come into question.”
Tsitsipas, 18, who stunned Rafael Nadal in the second round at Wimbledon, was taking on Murray for the second time in four days after becoming the youngest winner of a WTA main draw match since the Shanghai Masters in 2010.
Murray finished the American hard-court season number three in the world rankings, his highest position of the year, but was unable to back it up in Cincinnati.
He added: “I was very determined to have a big summer, which I did, but I know I’ve got a lot more to do and I’m nowhere near the level I’m going to have to be to win a grand slam championship.”
What were Murray’s key errors?
The Scot had a telling groundstroke error at 2-2 in the second set as he played for deuce.
However, there were encouraging signs on his forehand as he caused Tsitsipas plenty of problems, especially at the net.
But he was made to pay for several errant forehands of his own during the second set as he lost three straight games and fell a break behind in the decider.
Tsitsipas then broke at 4-3 before claiming a second successive break for the match in the 11th game.
The teenager, ranked 39 in the world, served out to clinch a superb victory.
Tsitsipas now heads to the hard-court event in Montreal where he has a bye in the first round and will face Frenchman Adrian Mannarino on Wednesday.