OAKLAND, Calif. (AP) Marcus Semien rounded third base on Josh Reddick's 10th-inning infield single, and Reddick's first thought was disbelief.
Then Semien slid home for the game-ending run not long after Oakland wasted a big opportunity in the ninth.
Semien scored from second base on Reddick's two-out infield single to lift the A's to a 4-3 win over the Houston Astros on Tuesday night.
''It was a really good game for us,'' manager Bob Melvin said. ''There was a lot of fight tonight, for sure. A lot of good things transpired.''
The A's rallied from three runs down and trailed 3-2 before Coco Crisp's tying double off Astros closer Will Harris in the ninth.
After the Astros stranded the bases loaded in the top of the 10th, Reddick won it with the sixth walkoff hit of his career.
Semien singled with two outs off Pat Neshek (2-2) and stole second. Yonder Alonso walked before Reddick singled to deep shortstop. Houston's Carlos Correa fielded the ball cleanly but threw wide of home as Semien slid safely in with the winning run.
''I was wondering why he was going and being sent in the first place,'' Reddick said of Semien. ''Then I saw the throw and I was like `Thank you, thank you, thank you.' I think he'd agree he had no business going. But sometimes you have to push the issue, and it worked.''
Crisp finished with two hits while Ryon Healy added two hits and two RBIs for the A's. Oakland has won four of five since the All-Star break.
Jose Altuve matched his career-high with four hits while George Springer hit his career-high 21st home run for the Astros.
''Definitely a winnable game,'' Houston manager A.J. Hinch said. ''Really this game was more about us not being able to close the game out. Late in the game it never felt like we had enough runs and it turned out we didn't.''
Mark Rzepczynski (1-0) got the last out in the top of the 10th for the win.
Houston starter Dallas Keuchel took a shutout into the seventh before giving up consecutive one-out hits to Billy Butler and Jake Smolinksi. Healy followed with a two-run double off reliever Ken Giles to pull the A's to 3-2.
Oakland tied the game in the ninth with back-to-back doubles by Stephen Vogt and Crisp. Crisp thought he had homered and slowed rounding second before getting caught in a rundown.
Altuve padded his majors-leading hit total with his sixth four-hit game of the season. Houston's All-Star second baseman singled in the first, doubled and scored in the third and tripled and scored in the fifth. He struck out looking in the eighth and singled again in the 10th.
Keuchel allowed two runs and five hits over 6 1/3 innings. The reigning AL Cy Young Award winner, who beat the A's in his final start before the All-Star break, struck out six and walked three.
A STEP FORWARD
A's starter Daniel Overton, who was called up from Triple-A Nashville before the game, allowed three runs over 6 1/3 innings. The rookie left-hander struck out six and didn't walk a batter for the first time in three career starts.
Altuve's three hits raised his AL-leading average to .352 and extended his road hitting streak to 18 games. The diminutive infielder has also reached safely in 34 consecutive road games, the second-longest streak in Astros history.
Athletics: Rich Hill's status remains in limbo because of a blister on the middle finger of his left hand. Hill left his most recent start Sunday after throwing just five pitches. ... RHP Andrew Triggs was placed on the 15-day disabled list with a bruised left shin. ... INF Tyler Ladendorf was optioned to Triple-A Nashville and LHP Daniel Coulombe was called up from the minors.
Astros: RHP Doug Fister (9-6) pitches the finale Wednesday afternoon at the Coliseum and has two wins and a 1.31 ERA in three starts against Oakland this season.
Athletics: RHP Daniel Mengden makes his second start against the Astros this season. He allowed six runs over 4 2/3 innings but took a no-decision in Houston on July 8.