After Josh Harrison struck out swinging, Marte lined an 0-1 pitch into the gap in right-center. Andrus scored without a throw when center fielder Jose Siri's throw back to the infield was low.
The A's started the day four games behind the New York Yankees for the second AL wild card. Houston's magic number to clinch the AL West remained at three over Seattle.
Andrew Chafin (2-3) retired three batters to win.
Harrison had an RBI single and Andrus singled twice for the A's.
Manaea struck out eight while pitching seven innings of three-hit ball. The A's lefty retired 19 of 21 after allowing Altuve's double on his first pitch.
Valdez gave up one run and three hits in 6 2/3 innings. He walked four and struck out three.
Tucker struck out swinging in his first two at-bats before sending a towering, solo drive off Manaea in the seventh for the Astros' second hit.
Oakland tied it in the bottom half on Harrison's two-out bloop single off Kendall Graveman that fell in just over the head of shortstop Carlos Correa. Graveman, the former A's starter, had just replaced Valdez.
Oakland's Matt Chapman, who tied a franchise record with five strikeouts on Friday, was in a 10 for 63 (.159) slump when he drew a one-out walk off Graveman to load the bases in the eighth ... Tony Kemp struck out looking as a pinch-hitter, ending a streak of 88 consecutive plate appearances without striking out. It was the longest streak by an A's player since 1974.
Astros: RHP Pedro Baez (shoulder soreness) is continuing his rehab assignment with Triple-A Sugar Land and was scheduled to pitch Saturday night.
Athletics: Harrison was back in the lineup after being forced out of Friday night's game when he fouled two balls off his lower left leg in the same at-bat.
Astros RHP Jake Odorizzi (6-7, 4.22 ERA) is expected to be activated off the 10-day injured list to start the series finale Sunday. Athletics RHP Paul Blackburn (1-3, 5.17) has allowed 13 earned runs in 6 1/3 innings in his two career appearances against Houston.
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