In the event of a retirement, disqualification, or change of surface during a match, all bets on these markets will be void unless the outcome is already guaranteed regardless of what happens next.
For example: if you bet Over 18.5 Total Match Games and a retirement occurs with the score at 7-6, 5-0, your bet is still a winner. Even if the match had continued to its natural conclusion, at least one more game was guaranteed, taking the total games to 19 or more.