The British world No 49 was on court for one hour, 48 minutes in registering a 6-3 7-5 victory over a player ranked seven places higher.
Watson recovered from 1-3 down in both sets and overcame a tense finale to set up a clash against either Madison Brengle or Kurumi Nara.
However, it was anything but plain sailing for Watson who missed three opportunities to seal victory in the 10th game of the second set when she held match points at 40-15 and then another on advantage.
But the 22-year-old responded by breaking serve in the next game and then held on to clinch a hard-earned triumph for the loss of only one point in game 12.
Watson’s only other final appearance came at Osaka in October 2012 when she won a tournament that was also staged on hard courts.
It was her second win over 24-year-old Riske in as many meetings and Saturday’s final will be between two unseeded players.
Nara of Japan, at 43 in the world, is the favourite to overcome her American opponent who is ranked 84.