1. It looks like Joanne is going to lose, but I wouldn’t be too sure. She may have an ace up her sleeve.
2. No matter how many times I think Paul might lose to me in a game of chess, he never does. He always has an ace up his sleeve and wins every game.

The expression originates from card games like poker, in which players might hide an extra ace up their sleeves to use in case they were losing the game and wanted to cheat.