With under 20 seconds left in the second period, Blues forward Pat Maroon grabbed a loose puck and searched for an open teammate. He fired a pass between the legs of Krejci that found a wide-open Pietrangelo in the slot.
Pietrangelo glided in close to the net and swerved to avoid Bruins defenseman Charlie McAvoy, who laid on the ice in an attempt to swat the puck away. Pietrangelo's swift move also fooled Boston goaltender Tuukka Rask, who fell to the ice in desperation.
At the last second, Krejci skated in front of the empty net and deflected Pietrangelo's wrist shot with his body.
The Blues held a 2-1 lead with under three minutes remaining in the third period. St. Louis' Ryan O'Reilly scored in the second period and David Perron added a score in the third. Bruins forward Jake DeBrusk also notched a goal in the third period.
The best-of-seven series is tied 2-2.