In an expected low-scoring, tight affair between two of the Eastern Conference's best defensive teams, it looked as if the match was headed to penalties.
But when the Red Bulls were unable to clear the ball, it chested off Glesnes, who positioned himself just outside the top of the box and curled his shot high past leaping keeper Carlos Coronel and into the corner of the net in the 123rd minute of the contest.
It was no surprise that scoring was at a premium in this contest. New York's 33 goals conceded during the regular season tied for fewest in MLS. Philadelphia, meanwhile, allowed just 35. The Union and Red Bulls met three times during the regular season, with five total goals scored.
New York had a pair of strong back-to-back chances early in extra time -- both from Patryk Klimala.
Alone from the right side, Klimala missed wide left in the 94th minute. Seconds later, his follow-up strike was stopped by Union goalkeeper Andre Blake -- New York' first shot on target of the match.
Sergio Santos nearly gave Philadelphia the lead early in the second-half of extra time when his well-timed, point-blank strike was abruptly stopped by Coronel. Shortly after, New York's Kyle Duncan nicked the far post on his shot.
These sides combined for 17 shots during regular time, with the Union recording the only one on target. But, it was a golden chance from Alvas Powell, whose point-blank strike was saved by Coronel in the first half.
Philadelphia stepped up the pressure about halfway through the second half. In the 82nd minute, Santos got in close and set up teammate Kacper Przybylko for a glorious chance, but the Union's leading scorer sent the ball well high over the net.
A streaking Santos had a side chance of his own in the waning seconds of regular time, but his strike landed into the hands of a standing Coronel.