In the event of two players reaching fifty after the same match, deciding matches are held between ALL competitors. If the deciding match is won by a CPU-controlled player or a non-deciding player (that is, a player who did not reach fifty and initiate the decider), play continues until deciding player wins a match (and hence the fifty). The winner of a decider is determined by overall score (KOs - deaths - SDs), not KOs.
The prefix 'quick' is typically used when only a single fifty is to be played, though this is sometimes said with an element of irony, as a quick fifty between players with a strong rivalry can (and often do) escalate into a series of fifties. Series are played with the score from the previous fifty retained, so losers are tasked with making up the difference in KOs during the next fifty. A series may be substituted for a deciding match, but never follow one.
"Argh, I'm so sick of losing fifties this week!"