Put somebody off their stroke
Definition 1 of 1
Pro Tip 1/3Sports Usage
Often used in sports to describe a loss of focus or nerves affecting performance. 
The shouts from the crowd really put the golfer off his stroke.
Pro Tip 2/3Distraction Cause
Emphasizes an external factor causing the distraction or disturbance. 
Loud construction noise outside the exam hall put me off my stroke.
Pro Tip 3/3Temporary Effect
Suggests that the effect on performance is not permanent, just momentary. 
Sorry for interrupting your piano practice, I didn't mean to put you off your stroke.