Sentence:  Official Knock up: if the players "knock up" together a maximum of five minutes shall be permitted; if they <WSD>knock up</WSD> separately each player shall be permitted to <WSD>knock up</WSD> for a maximum of five minutes. 
Meaning of the word marked with <WSD> tags:  To gently hit the ball back and forth before a tennis match, as practice or warm-up, and to gauge the state of the playing surface, lighting, etc. See '''knock-up'''. (from 19th c.)