Andy Murray will aim to find the answer to why he struggled to cope physically in his US Open defeat to Stanislas Wawrinka on Sunday night as another tilt at a first Grand Slam title ended in disappointment.
Murray took the opening set in a tie-break but Wawrinka hit back to complete a 6-7 (3/7) 7-6 (7/4) 6-3 6-3 victory in three hours and 56 minutes.
The match featured 13 breaks of serve and almost as many breaks in play for medical treatment to both players before Wawrinka sealed a fourth-round clash with American Sam Querrey.
Murray initially put any physical issues down to "just part and parcel" of playing a long match, but added: "Whether it was fatigue or not I don't know.
"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."
Murray did have stamina issues early in his career, but took determined action to rectify the problem, hiring a fitness team and undertaking strenuous fitness programmes in Florida at least twice a year.
The 23-year-old Scot added: "I still feel like I'm super fit, I just didn't feel great (today). There were a lot of things that I was feeling on the court. But, yeah, I just haven't felt that way for a few years now.
"I'm going to have to go look at why that was the case and try and get better."
Murray graciously did give credit to his friend and occasional practice partner Wawrinka, however, adding: "He played better than me. There's not a whole lot more to it.
"He had a chance to win the first set and he didn't take it. I had a chance to win the second set and I didn't take it. I just struggled from then on.
"I'm very disappointed, obviously. But I think I've been more disappointed. In other Grand Slams, when you get closer to winning the tournament, I think it becomes a lot harder to take."