Three minutes into the extra time, substitute Lenhart found the bottom left corner of the net on a header from the center of the box off a cross from Marvin Chavez, breaking a 1-1 tie.
Lenhart and Chavez also teamed up on the first goal of the game, which came in the 76th minute for the Earthquakes (6-1-1), who continued the best start in franchise history.
Gabriel Gomez found the net in the 83rd for the Union (2-4-1) with a right-footed shot from the center of the box on an assist by Danny Mwanga.
Philadelphia has managed only five goals in seven games so far this season.
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