Gain, get, have, etc. the upper hand
Definition 1 of 1
Pro Tip 1/3Power Dynamics
Used to describe a situation where someone gains control or dominance in a relationship, competition, or confrontation. 
After presenting our new strategy, we finally got the upper hand in the meeting.
Pro Tip 2/3Strategic Movements
Often used in contexts that involve strategy, like sports, business, or negotiations. 
The chess player gained the upper hand with a surprising move.
Pro Tip 3/3Temporary Advantage
Implies that the advantage may not be permanent; situations can change. 
They had the upper hand early in the game, but lost it by halftime.