Andy Murray crashed out of the US Open in the third round, being beaten in four sets by Switzerland's Stanislas Wawrinka.
The fourth seed started well and took the first set on a tie-break after recovering from an early break of serve, but then began to lose form and was overwhelmed by the powerful ground strokes of Wawrinka.
The Swiss twice recovered from a break down to take the second set on a tie-break, before powering home 6-3, 6-3 to reach the fourth round, where he will meet America's Sam Querrey.
Afterwards, Murray admitted that he had struggled during a match which lasted almost four hours.
He told reporters: "I haven't been really tired in any long matches for a long, long time.
"In the third and fourth sets I was struggling physically and I got frustrated with that. I haven't been in that position for a very long time.
"I felt like my chance of doing well here was slipping away. I've worked very hard to give myself a chance of winning tournaments. When I was struggling physically, I got disappointed."
Top seed and world number one Rafael Nadal had few problems in reaching the last 16 as he swept aside France's Gilles Simon 6-4, 6-4, 6-2.