Expression idiomatique en anglais : On the ball !

• knowledgeable and competent
• alert, in command of senses, attentive
• understands the situation well
• quick to understand and react to a situation

• He was right on the ball regarding his assessment of the situation.
• I couldn’t sleep well last night, so am not on the ball today.
• When his opponent tried to sneak in a pass, the defender was right on the ball and stopped it well.
• This mess wouldn’t have happened had you been on the ball.
• « Are you alright? You don’t seem on the ball today ».
• This article about the current problems of the nation is right on the ball.
• He has been sick all week and wasn’t on the ball when I met him.
• He has done a great job. He was really on the ball.

This phrase originated in sports, specifically in ball games where the players were asked to keep their « eyes on the ball ». The current usage of the phrase began in the 1900s.

Source: http://www.theidioms.com/on-the-ball/