Krejci scored 13:06 into the extra session to hand the Bruins a commanding 3-1 lead in the Easter Conference quarterfinal ahead of Game 5 in Boston on Friday evening.
Patrice Bergeron also scored for the visitors, who erased a 2-0 deficit with three goals in a wild second period before the Leafs tied it to send the game to overtime.
Krejci was humble about his heroics after the game and revealed he was just doing his job.
“I’m just trying to do my best,” he said. “I know I had a run a couple of years ago but I know I had a bad run so I’m just trying to play my best, go out there and do my hardest and not worry about the result.”
It was a heartbreaking loss for the Leafs, who jumped out to 2-0 lead in the first period thanks to goals from Joffrey Lupul and Cody Franzen.
However, Boston fought back with three goals in 17 minutes before Clarke MacArthur knotted things up to end a frenetic second period.
There was no scoring in the third period, which set up a free flowing overtime that left fans of both teams on the edge of their seats.
Toronto had a plethora of chances to win the game but was stopped time and again by Tuuka Rask and it was Dion Phaneuf’s mistake that led directly to Krejci’s overtime winner.
The Leafs’ captain inexplicably went for a thunderous hit that took him out of position, which led to a two on one that Boston capitalized on.
A solemn Phaneuf shouldered the blame for the gaffe that has put the Leafs in a big hole.
“It was a big mistake and a bad time to make it,” he said. “I take responsibility for it. It is unacceptable.”
David Krejci’s hat trick has put the Bruins in prime position to clinch the series against the Leafs at TD Bank Gardens on Friday.