For a favorite to win -2.5 sets, he must win in three straight sets. For the underdog to cover +2.5, he must win at least one set. Similarly, for a favorite to win -1.5 sets, he must win the match in four sets or better, 3-1 or 3-0. For the underdog to cover +1.5 sets, he must win at least two sets in a match.