off one's game

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• adjective sports, of a competitor Playing or competing below one's usual level of performance.
• adjective idiomatic, by extension Performing in any activity below one's usual level; behaving in an irregular, inept, or awkward manner; feeling unwell.
• adverb sports In or into a condition of competing below a usual level of performance.
• adverb idiomatic, by extension In or into a condition which reflects unaccustomed poor performance or unwellness in an activity or situation.