The Swiss star scored a gritty 7-6 (7-5) 3-6 6-2 win in Tokyo on Saturday to book a title clash with Czech player Linda Noskova, who advanced via a walkover.
Bencic will have another chance to win the trophy and erase bitter memories of her defeat by Poland's Agnieszka Radwanska in the 2015 final, and the Swiss player said she was looking forward to the challenge despite her exhaustion.
"I feel very old to hear that I'm in the final again after 10 years," Bencic said.
"It's really nice, of course. It's been a tough battle out there. I'm happy I was able to give it my all and go for it in the end. I'm really excited to play the final.
"Sometimes your mind convinces you to keep going and your body says, 'I don't want to', but you can find a way if you have to and you have the will to go through.
"Hopefully I can do that one more time."
Playing under the centre-court roof of the Ariake Coliseum because of rain, both players stayed strong on serve in the opening 12 games before Bencic turned up the intensity in a tight tiebreak to take the opening set with a forehand winner.
Former Australian Open champion Kenin came out firing to bag a break in the fourth game of the second set, then overcame a minor wobble on her own delivery in the next game to eventually win theset.
Bencic, the Tokyo Olympics gold medallist, broke for a 3-1 lead in the decider after some sloppy play by Kenin, and the 28-year-old rode the momentum to wrap up the victory and stay on course for her second title of the season.
"I think it was a very difficult match," said Bencic, who won the Abu Dhabi title in February, four months after returning to the circuit from maternity leave.
"I'm really exhausted at this point. I tried to shorten the points, take more risks and tried to go for it. Thankfully today it went my way."
Earlier, Elena Rybakina pulled out of her clash against Noskova with a back injury, a day after the Kazakh secured the last qualifying spot for next month's season-ending WTA Finals in Riyadh by reaching the semi-finals in Tokyo.
"I'm very sorry I can't play today," Rybakina said.
"I've been having problems with my back this week and can't play 100%. I'm disappointed that my fans will not see me today but I hope to see you next year."
Noskova, beaten in the final of this month's China Open by American Amanda Anisimova, is aiming to capture only her second WTA title, having triumphed in Monterrey last year.Â
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