1. be doing something very well, performing at one's highest level
Professional
Social
I'll need to be at the top of my game in the interview/match
When you are 'on top of your game' you are performing to the highest possible standard or performing at the height of your ability. This expression is often used in sport as well as in the workplace and used when someone couldn't possibly do any better. Similar in meaning to 'at the top of the ladder'.