Wiktionary
off one's game
a. 1 (context sports of a competitor English) play or compete below one's usual level of performance. 2 (context idiomatic by extension English) Performing in any activity below one's usual level; behave in an irregular, inept, or awkward manner; feeling unwell. adv. 1 (context sports English) In or into a condition of compete below a usual level of performance. 2 (context idiomatic by extension English) In or into a condition which reflects unaccustomed poor performance or unwellness in an activity or situation.