Revolution began the game in attacking fashion and reaped the rewards when Lee Nguyen was taken down in the box.
The midfielder stood up to take the penalty himself, firing past Andre Blake to give his side the only goal of the first half.
It was the on-loan Nantes forward who levelled matters for Union in the second half with a near-post finish that Bobby Shuttleworth was unable to keep out.
Jermaine Jones attempted to reinstate Revolution's lead with a 30-yard strike but Blake tipped the effort over his crossbar.
Kelyn Rowe tested the Union keeper moments later but he again stood firm to keep his side on level terms.
Sebastien Le Toux responded with an effort for Philadelphia but Shuttleworth denied the French forward.