The third-seeded Crew host No. 8 New England in the conference final on Dec. 6.
Santos broke a 309-minute playoff goalless streak by Nashville goalkeeper Joe Willis when he slotted the cross from the left side by Zardes.
Zardes made it 2-0 on the counter with a timed run to take the ball from Diaz to get behind the defense.
Andrew Tarbell, who had two shutouts in seven games this season when Room was injured, got the start. Diaz took Etienne's spot on the right flank.
Only family members and friends of the Crew players and staff could attend the match at Mapfre Stadium.
Willis didn't need to make a save until the 79th minute but it was an outstanding reaction stop on Zardes. Willis made three saves; Tarbell had four.
The Crew had the better of the play from that point to the end of regulation as they took advantage of Nashville playing its third postseason match in 11 days, including a 1-0 extra time win over Toronto FC in the quarterfinals on Tuesday.
Columbus had a week off since defeating the New York Red Bulls, but Nashville had the early chances.
Jhonder Cadiz had a header that forced Tarbell to make his first save in the 24th minute and three minutes later, Dax McCarty sent a header from a corner kick off the crossbar. The ball fell to Walker Zimmerman, but his heel shot was stopped by Tarbell at the left post.