Matin Prado singled to lead off the 10th against Bobby Parnell (4-4) and went to second base on a groundout.
Freddie Freeman was then handed an intentional walk and Dan Uggla coaxed an unintentional base-on-balls to load the bases, and McCann followed with a fly ball to right field for the go-ahead tally.
Craig Kimbrel (1-1) earned the victory with a scoreless top of the 10th and Peter Moylan collected his first save despite allowing a pair of runners in the bottom of the frame for National League wild card-leading Atlanta.
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