Default chances change as you win matches to upgrade the rarity of your pet. Each win provides a small boost to your win chance. For example, a pet with 1 win would have 0.49 more chance to win compared to a pet win no wins. This boost, however, is not fixed and you will get less bonus chance to win with each consecutive win. If your opponent has the same number of wins, it would be 50/50 win or loss. Check the table below for some details: