The Australian ace opened the scoring for the Red Bulls early in the match, and provided the second, but a second-half red card to Jamison Olave allowed the Houston Dynamo to force a 2-2 draw at the BBVA Compass Stadium.
Former Everton star Cahill netted his 12th goal of the season - and his first post-season MLS tally - when he got on the end of a pinpoint Thierry Henry delivery from the left to head home from the edge of the six-yard box in the 22nd minute.
The Red Bulls went two ahead just after the half-hour mark when Cahill received a long ball from Dax McCarty in midfield, turned and threaded it in front of Eric Alexander, who advanced into the area before slotting home.
However, the Red Bulls two-goal was cut back early in the second half through Ricardo Clark before Olave was given his marching orders in the 65th minute allowing Omar Cummings to net an injury-time equaliser.
New York and Cahill will look to progress to the next phase of the 2013 MLS playoffs when they host the Dynamo in the return leg on Wednesday.
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