put (one) off (one's) stroke

To disturb, distract from, or interfere with what one is doing, such that they are unable to do it well. Having my parents watching while I worked on my painting really put me off my stroke. The boxer tried to put his opponent off his stroke by getting in his face right before the fight.

put someone off their stroke

BRITISH

If something puts you off your stroke, it stops you from concentrating on what you are doing, so that you do not do it as well as usual. A sudden sound from behind me put me off my stroke and I missed the ball completely. Compare with put someone off their stride. Note: The reference here is to rowing, in which all the members of a team have to pull on their oars at exactly the same moment. Each pull of the oars is called a stroke.

put someone off their stroke

disconcert someone so that they do not work or perform as well as they might; break the pattern or rhythm of someone's work.