Hellickson (1-1) allowed two hits and one run in seven innings, struck out three and walked none. He hadn't reached five innings in any of his previous three starts this year or in five outings dating to the end of the 2013 season. Hellickson returned late last month following right elbow surgery in January.
The Rays handed reigning AL Pitcher of the Month Sonny Gray (12-5) his second straight defeat following a 5-0 July in which he had a 1.03 ERA. This is the second time Gray has lost consecutive outings in his first full major league season, also to Baltimore and the Yankees on June 7 and 13.
Kiermaier's first-pitch homer in the fourth was just the ninth surrendered this year by Gray, who came into Wednesday with a 1-0 record and 0.61 ERA in two career starts against the Rays.
Hellickson retired the first nine A's hitters in order before Coco Crisp's leadoff single in the fourth, then the right-hander recorded seven more outs in a row.
Eric Sogard broke that up with a sixth-inning homer, snapping a career-high 110-game homerless streak dating to July 21, 2013. Josh Reddick added a two-run double in the ninth off former A's closer Grant Balfour.
Gray plunked Kiermaier in the fifth to load the bases and Desmond Jennings followed with a two-run single to chase the right-hander after a season-low 4 1-3 innings. Jesse Chavez, moved to the bullpen after last week's Jon Lester trade, worked 3 2-3 innings.
The Rays took advantage of opportunities after going 1 for 15 with runners in scoring position in the first two games of the series.
Crisp returned to center field following a game at designated hitter as he worked back from missing seven games with a strained neck.
Rays: Catcher Jose Molina took a foul tip on his right arm in the third but stayed in the game. ... Outfielder Wil Myers, on the 60-day disabled list with a broken right wrist, was scheduled to report to Triple-A Durham to continue rehab work and baseball activities. He could begin playing in minor league games by the weekend.
Athletics: Manager Bob Melvin expects shortstop Jed Lowrie back from a bruised right index finger for Thursday's series opener with the Twins.
Rays: After an off-day in Chicago, RHP Chris Archer (7-6, 3.42 ERA) pitches Friday's opener of a weekend series at Wrigley Field to close out interleague play.
Athletics: LHP Lester (11-7, 2.59) makes his second start since being traded to the A's from the Red Sox last Thursday. He is 5-0 with a 1.38 ERA over nine starts since a June 7 loss at Detroit.
The Oakland-Alameda County Coliseum Authority approved the Athletics' 10-year lease deal that keeps the club in Oakland for now — a formality in the deal reached last month.