Lorenzo Cain's bases-loaded walk in the 10th inning scored Alex Gordon with the deciding run, as Kansas City took a 5-4 decision over Boston in the back end of a doubleheader from Fenway Park.
George Kottaras, Gordon and Billy Butler each slugged a solo homer for the Royals, who swept both ends of the twinbill, following a 4-2 decision in the opener.
"I thought we had great at-bats in crucial situations," Royals manager Ned Yost said.
Kelvin Herrera (2-2) fanned three and walked one while allowing one hit in two scoreless relief frames. Greg Holland earned his fifth save by turning in a perfect bottom of the 10th.
Mike Napoli homered among his three hits and drove in two runs for the Red Sox, who had won seven in a row prior to Sunday. Dustin Pedroia added two hits and an RBI in defeat.
Andrew Miller (0-1) gave up the lead run in his lone frame on the hill.
Miller allowed a one-out single to Gordon, then walked Alcides Escobar before fanning Butler. Sox shortstop Pedro Ciriaco's dive at deep short to smother an Eric Hosmer grounder saved a run but loaded the bases ahead of a walk to Cain which forced in Gordon to make it 5-4 for Kansas City.
Holland then struck out the side to lock up the victory.
Gordon started the game with a double and scored when Escobar followed with a single to deep short.
Napoli began the home second by clearing the Green Monster, then Mike Carp drove in Jarrod Saltalamacchia with a double for a 2-1 Sox edge.
Pedroia plated Jacoby Ellsbury with a third-inning single, but Kottaras launched a pitch into the bullpen for a one-run contest and Gordon's high line drive to left made it 3-3 in the fifth.
Napoli plated Nava with a single in the sixth, then Butler's deep drive to left center tied the game in the eighth.
Boston stranded two on in the bottom of the ninth.
Though KC has won two of three from Boston this season, the Red Sox have won the season series in every year since 2010 ... Gordon finished 3-for-5 and scored three runs ... Ellsbury hit safely twice in defeat ... In his major- league debut after being recalled from Triple-A Pawtucket, Allen Webster allowed five hits and three runs -- two earned -- with five strikeouts and one walk in six innings ... Royals starter Jeremy Guthrie lasted six frames and yielded eight hits and four runs with four walks.