Knock Out is one of the battle mode competitions in Sonic Rivals 2 played in Act 2 of any Zone. During the story of all playable characters, it is the most common battle mode competition played against the player's rival.
Each character starts with only three Rings. The player's objective is to first attack his/her rival, using either attacks, Power-Up or falling into obstacles, to knock off their Rings and then attack them again (whether it is by attacks, power-ups or obstacles) to win the round. Note that the player's rival will be attempting to do the same and as such, the rival character will mainly try to pursue and attack the player's character in order to defeat him/her. In this mode, there is a star bar next to each character's icon which keeps track of the number of rounds the player has won. Each time a player wins a round, a star will light up. The winner of this competition is the character who can win a specific number of rounds first. (In Free Play, the number of winning rounds can be set by the player to one, three or five).