Ymer upset the only player among the four semi-finalists who previously had reached a final, No.15 seed Carlos Alcaraz, by a score of 7-5 6-3 as he contested his first ATP semi-final at the North Carolina event.
The 22-year-old Swede, ranked No.90 in the world, knocked off Alcaraz in the second round at this year's Australian Open in their only prior meeting.
Ymer converted all three of his break points against the Spanish teen, who is ranked a career-best No.54 this week.
Ivashka had no trouble disposing of Emil Ruusuvuori 6-2 6-1 in the first semi-final.
The Finn had won 93 per cent of his service games this week entering the match but was broken in four of eight on Saturday (AEST).
Ivashka, meanwhile, did not face a single break point on his own serve as he cruised to victory in 68 minutes.