The 35-year-old two times Wimbledon champion, a former world number one currently ranked 52nd, bowed out in the second round after a four set defeat to big-serving American John Isner, the 20th seed.
"I could have had a good run here," he said. "One of the reasons why improving your ranking and trying to get seeded is important, (is to) avoid playing top players and dangerous guys like that early in tournaments.
"It's one of those matches that, had I got through, who knows what would have happened," added the Scot. "It's frustrating for different reasons."
