The Bruins have not lost a playoff game in Toronto since 1959.
Joffrey Lupul, Cody Franson and Clarke MacArthur scored for Toronto. Patrice Bergeron added a goal for Boston, with captain Zdeno Chara collecting four assists.
Goalies Tuukka Rask and Reimer were both busy in an end-to-end overtime, with Toronto's Matt Frattin hitting Rask's goalpost.
Krejci has five goals and five assists in the series, combining with linemates Milan Lucic (0-7) and Nathan Horton (3-3) for eight goals and 15 assists.
Kings 3, Blues 2 (OT) • In St. Louis, Slava Voynov scored on an odd-man rush eight minutes into overtime and the Los Angeles Kings beat the St. Louis Blues for the third straight time, 3-2 Wednesday night.
The win put the defending Stanley Cup champions on the verge of surviving the first round, leading 3-2 heading to Game 6.
Rangers 4, Capitals 3 • In New York, Dan Girardi and Derek Stepan scored third-period goals for the York Rangers, who squandered a two-goal lead and then held on to get even in the best-of-seven series.
Girardi ripped a shot from above the left circle, off a feed from Derick Brassard, to give the Rangers the lead again with a power-play goal 59 seconds into the third. The advantage was created by Jason Chimera's interference penalty at the end of the second.
Ducks 3, Red Wings 2 (OT) • In Anaheim, Calif., Nick Bonino scored 1:54 into overtime, and Anaheim moved to the brink of the second round with a win over Detroit.
Defenseman Ben Lovejoy took the puck behind the Detroit net and fed it in front. Bonino scored for the Ducks, who took a 3-2 series lead.
• Boston 4, Toronto 3 (OT)
Bruins lead series 3-1
• N.Y. Rangers 4, Washington 3
Series tied 2-2
• Los Angeles 3, St. Louis 2 (OT)
Kings lead series 3-2
• Anaheim 3,Detroit 2 (OT)
Ducks lead series 3-2
• N.Y. Islanders at Pittsburgh, 5 p.m., CNBC
Series tied 2-2
• Ottawa at Montreal, 5 p.m., NBCSN
Senators lead series 3-1
• Minnesota at Chicago, 7:30 p.m., NBCSN
Hawks lead series 3-1