Britain's Alfie Hewett was beaten 6-2 4-6 6-3 by France's Stephane Houdet in the US Open wheelchair singles final.
Hewett had hoped to add the title to the doubles trophy he won on Saturday.
But the 19-year-old, who won his maiden Grand Slam singles title at the French Open this summer, succumbed to fatigue.
Hewett beat compatriot Gordon Reid in a three-hour semi-final on Saturday before teaming up with the Scot to defeat 46-year-old Houdet and Nicolas Peifer in the doubles final.
Fellow Briton Andy Lapthorne lost 7-5 3-6 6-4 to his American doubles partner David Wagner in the quad singles final.
It came after the pair had earlier claimed the quad doubles title with a 7-5 6-2 win over Australia's Dylan Alcott and Bryan Barten of the US.
It was a repeat of Wagner's victory over Lapthorne in August's British Open final.
"Not to be for me today in the singles," Lapthorne tweeted.
"Fair play to Wags, he was brilliant at the end. I'll be back."
David Wagner also beat Lapthorne at this year's British Open Hewett was also gracious in defeat.
"Congratulations to Steph for a great final," he said. "I just didn't have enough in the tank after such an exhausting singles and doubles yesterday.
"But I'm proud to be in the final in my first time here. There's a lot of positives to take and hopefully I can be back next year."