I don't get why 'shape' is used here instead of 'condition'? 'in the best condition' sounds more suitable to me.

Native speaker’s answer
Rebecca
Good point. Both 'to be in the best shape' and 'to be in the best condition' have the same meaning and are used equally in English. Ex: My car is not in the best shape, it's 15 years old. Ex: I drank way too much last night, so I'm not in the best condition today.