Andy Murray says it is "completely normal" to have doubts about whether he can return to the top of the game after his Grand Slam return ended in a second-round defeat at the US Open.
The British former world number one lost in four sets to Spanish 31st seed Fernando Verdasco at Flushing Meadows.
Murray, 31, was playing best-of-five matches for the first time since hip surgery in January.
"You just don't know exactly what's round the corner," he said.
Murray, a three-time Grand Slam champion, has only played five tournaments since returning to the ATP Tour in June after an 11-month absence with the hip problem.