Pirates 3, Nationals 1: Andrew McCutchen went 3-for-4 with a solo home run and A.J. Burnett struck out nine over seven frames as Pittsburgh downed visiting Washington.

Jordy Mercer, who was called up from the minors earlier Friday, delivered the go-ahead two-run homer in the fifth, giving Burnett (3-2) all the margin he needed. Burnett improved to 8-3 all-time against the Nationals while Jason Grilli struck out the side in the ninth for his 12th save in as many chances.

Danny Espinosa’s RBI single in the fifth accounted for the Nationals’ only run, continuing Washington’s recent offensive struggles, particularly with Ross Detwiler (1-3) on the mound. The Nationals have scored three runs or fewer in six straight games and managed a total of three runs over Detwiler’s last three outings.

McCutchen, who entered Friday batting .193 against left-handers, continued his mastery of Washington in the first inning when he drove a 2-1 fastball from Detwiler into the second deck of PNC Park. The two-time All-Star is batting .456 with 11 homers and 22 RBIs in 24 career games against the Nationals.

Washington evened it in the fourth when Espinosa followed an Adam LaRoche walk and a Tyler Moore single with his first RBI in six games. But Mercer staked Burnett to a 3-1 lead in the fifth when he lifted Detwiler’s 2-0 fastball just over the fence in left.

GAME NOTEBOOK: Washington 3B Ryan Zimmerman returned from the disabled list after a 14-game absence, but struck out in all four at-bats. …. The Pirates improved to 5-8 when scoring four runs or fewer. … Nationals OF Jayson Werth sat out Friday and is expected to miss the rest of the series due to a hamstring injury. Manager Davey Johnson stated before the game that he hopes Werth can return Tuesday and that he will not be placed on the disabled list.