BOSTON >> The Boston Red Sox beat Tampa Bay 6-4 tonight on a walk-off, two-run homer by Christian Vazquez in the 13th inning to move one victory from eliminating the 100-win Rays from the AL Division Series.

The wild-card Red Sox took a 2-1 edge in the best-of-five matchup. Game 4 is Monday at Fenway Park — Marathon Day in Boston — with Game 5 in St. Petersburg, Florida, on Wednesday, if necessary.

Tampa Bay rallied from a 4-2 deficit to tie it in the eighth inning and it was still 4-all when Yandy Diaz singled with one out in the 13th.

Kevin Kiermaier lined a two-out shot to right-center that bounced off the short wall in front of the Red Sox bullpen, caromed off right fielder Hunter Renfroe and flew back into the bullpen as Renfroe leaped to try to keep it in play.

It’s a good thing he didn’t: After the umpires conferred and went to the headsets, they correctly awarded Kiermaier a ground rule double and sent Diaz, who had come all the way home, back to third.

Diaz was halfway from second to third and easily would have scored if the ball remained in play.

But baseball Rule 5.05(a)(8) states: “Any bounding fair ball is deflected by the fielder into the stands, or over or under a fence on fair or foul territory, in which case the batter and all runners shall be entitled to advance two bases.”

Nick Pivetta struck out Mike Zunino to end the inning, then came bounding off the mound in celebration.

Renfroe walked with one out in the bottom half, then Vazquez hit the first pitch Luis Patiño over the Green Monster to end it.

Kike Hernandez and Kyle Schwarber each had three hits, including a homer, for Boston.

“It doesn’t make sense to me,” Kiermaier said after it was over. “It’s a heartbreaker, plain and simple