Nate Schierholtz went 2-for-5 in his Phillies debut.
Jimmy Rollins tied the game with a solo shot to lead off the fifth inning at Nationals Park, and Schierholtz made it back-to-back home runs to give the Phillies a 3-2 lead.
The final score: Phillies 3, Nationals 2.
The Giants traded Schierholtz to Philadelphia on Tuesday for two-time All-Star Hunter Pence.
Schierholtz appeared in 503 games for the Giants from 2007 to 2012, batting .270 with 23 home runs and 119 RBI.
He was drafted by the Giants in the second round of the 2003 MLB Draft.