put someone off their stride
put (one) off (one's) stride
To disturb, distract from, or interfere with what one is doing, such that they are unable to do it well. It really put me off my stride to have my parents watching while I worked on my painting. The boxer tried to put his opponent off his stride by getting in his face right before the fight.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
put someone off their strideBRITISH
If something puts you off your stride, it stops you from concentrating on what you are doing, so that you do not do it as well as usual. It was clearly a tactic designed to put his opponent off his stride. Note: The verbs knock and throw are sometimes used instead of put. Perhaps a few jokes during the game will knock Chris off his stride. Compare with put someone off their stroke.
Collins COBUILD Idioms Dictionary, 3rd ed. © HarperCollins Publishers 2012