KANSAS CITY -- Dominic Smith’s speed may have turned the tide of Boston's playoff push.

The 224-pound first baseman isn’t known for his legs, or his ability to swim around the tag, but Smith displayed both to steal a crucial run in Boston’s 6-5 series-clinching win over the Royals at Kauffman Stadium on Tuesday. The victory clinched the tiebreaker over the Royals, with Boston taking four of five from Kansas City with one to play, and moved the Red Sox to a half-game game behind K.C. for the third and final AL Wild Card spot.

But without Smith racing down the third-base line, none of Boston’s movement in the standings may have transpired.

Smith stood at third with the score tied in the sixth when Royals starter Seth Lugo threw a curveball in the dirt, which got past Salvador Perez. Smith initially froze, unsure if he would break for home, but once he saw the ball destined for the backstop, Smith took off. Perez handled the ball and threw to Lugo in time to make a quick tag, but Smith turned his body, picked up his left arm and slid his right arm toward the plate.

The initial call was that he was out, keeping the game tied and leaving a runner on second with two outs, but it was overturned after review, giving the Red Sox a lead they would never relinquish.

Masataka Yoshida then added the final blow, a two-run single to right, to cap his three-RBI night one day after a four-hit performance in the opener. Yoshida has been one of the Red Sox’s biggest bats since returning from a right thumb sprain injury in June, hitting .324 with an .893 OPS in his past 30 games prior to Tuesday.

Boston entered the night with a .309 average and .921 OPS in 16 games since the All-Star break, and has now scored 15 runs in the first two games in Kansas City.

Kenley Jansen had to come in for a four-out save after an eighth inning rally from the Royals, and the veteran closer shut the door. The Red Sox can move into a playoff position and earn the sweep Wednesday with Kutter Crawford slated to take the mound.