Zverev moved a match closer to repeating as champion with a 6-2 6-4 victory against fellow young gun Stefanos Tsitsipas.
Germany's Zverev, the oldest of the last four at 21, will play Sunday for the $384,120 (331,637 euros) top prize against the later victor between Russian Andrey Rublev, 20, and 19-year-old Australian Alex De Minaur. Zverev edged it, winning a deciding set tiebreak to triumph 7-5 4-6 4-6 6-3 7-6.
He saved four consecutive match points from 2-6 down in the second-set tie-break before pulling out the win, 5-7, 7-6 (6), 6-4, after nearly three hours on court.
The Russian broke twice in the decider, but de Minaur converted his fourth match point when Rublev double faulted. The 19-year-old Carlos Moya lifted his first ATP title, defeating the 21-year-old qualifier Felix Mantilla 6-0 6-3 in just 59 minutes.
"I don't think Roger is too concerned about it", Zverev said.
The top-seeded German is looking to become the first back-to-back victor of the hard-court US Open tune-up since Juan Martin del Potro in 2008 and 2009.
Zverev won the match 6-2,6-4.
"Andy is on his way back", Zverev said.
"I had to play some really good points to get myself out of there".
The Citi Open, which takes place in Washington, is an ATP circuit tournament and is endowed with $1.89 million in prize money.
Rublev, who won his only ATP title a year ago at Umag, beat American Denis Kudla 6-1, 6-4, in a quarter-final delayed from Friday by rain.
However, his only previous meeting with the slight Australian de Minaur was a marathon effort for both men, a Davis Cup five-setter grabbed by the tall German in a final-set tie-breaker-and played on hard courts.
"He's got a massive serve", de Minaur said.
"But tomorrow's going to be a very tough match against a very high-quality opponent and I'm just going to try and recover and leave it all out there again".
The third set broke things open as both exhausted players traded breaks, there were 5 in total with the last two going in favor of ADM as he recovered from a break down early in the third to break in the last game of the match on his fourth match point to reach the final.
Fifth-seeded Mihaela Buzarnescu will be gunning for her first WTA title on Sunday when she takes on Maria Sakkari in the final at San Jose, California.
The 2004 US Open and 2009 French Open victor will face seventh-seeded Donna Vekic of Croatia or unseeded Zheng Saisai of China as she looks for her 18th WTA title.
It was an 18th career tour title for the 33-year-old two-time Grand Slam champion, who had slipped to 128th in the world heading to Washington.
"Russia and America politically, it's probably not so good, but it's great people in sport are outside of this", Kuznetsova said.