Josh Donaldson hit a two-run homer to back Dan Straily's strong start, and the Oakland Athletics beat the San Francisco Giants 4-1 in the Bay Bridge Series opener Monday.

Straily (3-2) tossed six innings of one-run ball, allowing four hits and one walk. Buster Posey's groundout in the sixth inning drove in San Francisco's lone run.

Donaldson connected against Madison Bumgarner (4-3) in the fourth, and Yoenis Cespedes added a two-run double against George Kontos in the seventh to extend Oakland's lead.

Sean Doolittle pitched two perfect innings and Grant Balfour recorded his 11th save of the season for Oakland, which has won four straight and nine of 10.