The two teams battled through 12 scoreless innings until the Cubs scored twice in the top of the 13th. The Padres responded in the bottom of the inning, using an RBI triple from Ronnie Cedeno and a wild pitch by Kevin Gregg to tie the score at 2-2.
It stayed that way until the 15th, when Forsythe stroked a one-out single off Hector Rondon (2-1) and went to second on Cedeno's groundout. The Cubs intentionally walked Alexi Amarista to get to Hundley, but the Padres catcher lined a single to center, chasing home Forsythe with the game-winning run.
Dale Thayer (2-3) worked out of jam in the top of the 15th to earn the decision in 5-hour, 13-minute marathon as the Padres took the rubber match of a three-game series.
San Diego starter Andrew Cashner hurled seven shutout innings, allowing just two hits, while Cubs counterpart Chris Rusin tossed six scoreless innings, yielding three hits.