A's starter Jarrod Parker fared much better, allowing just one hit over two efficient innings in his Cactus League debut, throwing 18 of his 22 pitches for strikes. He struck out two in the first, both looking.
In turn, his teammates went to work against Giants lefty Madison Bumgarner, who faced 10 batters over two innings, giving up a run in the second frame on an RBI hit off the bat of second baseman Jemile Weeks, who is 6-for-11 this spring.
San Francisco stormed back for four runs in the third, though. With lefty reliever Jerry Blevins on the mound for the A's, Peguero, Kensuke Tanaka and Jackson Williams collected consecutive singles to tie the game. Angel Pagan's potential double-play ball was then bobbled by Weeks for an error, allowing another run in.
Marco Scutaro's sacrifice fly extended the Giants' lead, and after Blevins struck out Cole Gillespie for the second out, watched another run come in on Belt's RBI base hit, leading to the lefty's departure.
A's outfielder Chris Young, sidelined since Sunday because of a quad cramp, went 2-for-3 in his return to game action.
Starting with the top of the sixth, each team scored at least one run in six of the eight half-innings, with the A's getting as close as two runs down before the Giants twice stretched the lead back out to four, the final time coming on Angel Vilalona's solo homer in the top of the ninth.
Up next: The Giants split up Saturday, hosting the Cubs and traveling to play the Royals. Tim Lincecum gets the start vs. Chicago, while Boof Bonser is slated to be the first out of the gate against Kansas City. Both games start at 12:05 p.m. PT.
Jane Lee is a reporter for MLB.com. Read her blog, Major Lee-ague, and follow her on Twitter @JaneMLB. This story was not subject to the approval of Major League Baseball or its clubs.