I also struggled with the def whilst doing the crossword, couldn't find a reference linking "Davis Cup" and "rubbers", I always feel clues that assume a specialised knowledge of a particular field to be unfair to anyone with no experience of it.
Is this right place to ask about another clue in same crossword? 7 down. “Bed you’ll find in Washington DC’s superior. Answer WADI which is a bed but I can’t parse it. Help please