The Mystics had a one-point lead with 45 seconds left after Tierra Ruffin-Pratt went 1 for 2 at the line. But Pierson made a jumper and Skylar Diggins hit a pair of free throws to put Tulsa (5-1) up by three. Washington's Kara Lawson missed a 25-footer, and Pierson made a free throw with 1 second left.
Pierson was 9-of-19 shooting, and Williams hit 4 of 8 3-point attempts.
Emma Meesseman led the Mystics (3-2) with 24 points and 10 rebounds. Stefanie Dolson had 11 points and nine boards. Washington was 4 of 20 from beyond the arc.
The Mystics beat the Shock in their previous four meetings.