get the upper hand

To come to be in a position of advantage, power, or control (over some person, thing, or situation). We've been doing everything we can for your father, but I'm afraid the cancer is getting the upper hand. The home team got the upper hand when their opponents' star quarterback went out with an injury. The underdogs got the upper hand in the first half of the game, but the former champions overtook them after halftime.

get, have, gain, etc. the ˌupper ˈhand (over somebody)

get, etc. power or control over somebody, especially in a fight, competition, etc: Our team gained the upper hand in the second half of the match. ♢ The police claim they have the upper hand in their fight against the drug dealers.