Stephens, who is going to play her first final in Singapore, also expressed her happiness as she has outshined some great players in the tournament.
After earning championship point, she took it when Stephens produced another unforced error.
The WTA Finals ends its five-year run in Singapore this season and moves to Shenzen, China for a 10-year run starting in 2019.
Following the turnaround victory, Stephens will now face world number seven Elina Svitolina in the finals of the WTA Finals later today.
Timea Babos and Kristina Mladenovic played a masterclass to upset world number one Barbora Krejcikova and Katerina Siniakova in the WTA Finals doubles decider Sunday.