David Ortiz put Boston 2-1 up in the bottom of the sixth with a two-run shot off Cardinals phenom starter Michael Wacha, who had previously only conceded one run in the whole of the post-season.
But after Red Sox starter John Lackey struck out Allen Craig to start the seventh, a walk to David Freese and Jay's single convinced Boston manager John Farrell to call Breslow from the bullpen.
A double steal but runners on second and third and when Matt Carpenter launched a fly ball to left, catcher Jarrod Saltalamacchia could not handle Jonny Gomes' throw, ensuring Pete Kozma, pinch running for Freese, scored on the sacrifice fly.
Breslow collected the ball behind the plate and attempted to get the out at third, but his throw was wild and Jay came round to give the Cardinals a 3-2 lead, before Carlos Beltran drove in another to add an insurance run.
Yadier Molina's RBI ground-out in the fourth which followed Matt Holliday's lead-off triple had put the Cardinals ahead before Ortiz launched his second homer in consecutive nights to give the Red Sox a brief lead.
The best-of-seven series heads to St Louis for game three on Saturday.