Giants 4, Mets 2: Buster Posey hit a go-ahead two-run homer and Brandon Crawford drove in a pair of runs as host San Francisco took the opener of the three-game series.

Matt Cain came off the disabled list and allowed two runs on three hits in seven innings while Jeremy Affeldt (1-1) picked up the win in relief on his 35th birthday for the Giants, who won for the eighth time in 10 games. Sergio Romo worked a 1-2-3 ninth for his league-leading 19th save.

Posey came up with a runner on second and one out in the eighth and blasted a 1-0 cutter from Carlos Torres (2-3) into the stands in left-center field to put San Francisco on top. Jonathon Niese battled Cain to a draw with two runs allowed in seven frames and Daniel Murphy hit a two-run homer for the Mets.

Cain took a no-hitter into the sixth before Ruben Tejada led off with an infield single to deep short. Tejada was erased on a double play and Cain faced the minimum until Matt den Decker led off the seventh with a double and Murphy followed by taking a 1-0 slider just over the high wall in right-center to give the Mets a 2-1 lead.

Brandon Hicks tripled and scored on Crawford’s sacrifice fly to give the Giants their first run, and the duo hooked up on the tying run again in the seventh. Hicks walked with two outs and moved to second on a wild pitch before coming across on Crawford’s single to center.

GAME NOTEBOOK: Mets OF Eric Young Jr. (hamstring) reportedly suffered a setback in his rehab and will be pushed back. … New York top prospect RHP Noah Syndergaard (shoulder) had X-rays and a CT scan come back negative Friday and will resume baseball activities in a week. … San Francisco designated LHP David Huff for assignment in order to clear a roster spot for Cain.