Learner Tien defeated close friend Alex Michelsen to reach the NextGen ATP Finals title match on Saturday but admitted he "hated" having had to face his American compatriot.
The 19-year-old left-hander, ranked at 122 in the world, came through 2-4, 4-2, 1-4, 4-0, 4-1 in a semifinal which boasted tennis legend Rafael Nadal among the spectators.
"I can't put it into words right now," said Tien who hit 21 winners past the 41st-ranked Michelsen.
"I am really happy with the win but I hate it against him, but that is just how it goes. I started smiling when I came out here and saw him on the other side of the net, but I am really happy to get through and think I played really well."
Tien will face Brazil's Joao Fonseca, the lowest-ranked player in the draw at 145, who eased into the final by seeing off France's Luca Van Assche 4-2, 4-2, 4-1.
Fonseca is the second youngest man to make the NextGen final after current world No 1 and two-time major winner Jannik Sinner who took the title in 2019.
The Brazilian has won all of his four matches in Jeddah this week, a run which included the biggest win of his career against 20th-ranked Arthur Fils of France.
"Today was a great match. I played really solidly," said Fonseca after his one-hour victory.
"He didn't have too many chances because I was playing so aggressively and making all the shots. There is one more match to go, and I hope I can for it."
Sunday's final will feature the two youngest players in the eight-man field with Fonseca having the advantage of already beating Tien during the group stages.