The game pitted the Crew's MLS Cup championship against the holder of the 2020 best regular season record (Supporters' Shield) before the limited capacity sellout crowd of 5,180.
An entertaining first half got off to a rousing start in the first 30 seconds when MLS Cup MVP Lucas Zelarayan sprung Diaz for a shot that Blake stopped. Blake six minutes later denied Zelarayan's targeted shot for the left corner.
The Union tested Room in the 19th minute when he made an acrobatic save on a header Kacper Przybylko.
It was Zelarayan's turn 15 minutes later with a dash through the defense and a dipping shot off the crossbar. The Union immediately came down and Jamiro Monteiro's shot from outside the box went straight to Room.
The match was the final opener at Historic Crew Stadium (formerly Mapfre Stadium).
The last of four home games there is June 19 vs. Chicago before the $314 million new Crew Stadium opens July 3 against New England, the same opponent on May 15, 1999, when the current venue was christened as the first soccer-specific stadium in MLS at a cost of $28 million to then-owner Lamar Hunt.
The Union hosts Inter Miami CF on Saturday. The Crew's next MLS match is May 1 vs. CF Montreal in Fort Lauderdale, Fla.