Naomi Osaka in a battle between the last two U.S. Open champions at the WTA Finals on Monday before top seed Angelique Kerber was upset by Kiki Bertens.
There Osaka's momentum ended, as she swiftly found herself 1-4 down to a more inspired Stephens.
"I think that's one thing I can be really proud of today, that I turned that match around, that I played more aggressive, going a little bit out of my comfort zone and to get a win".
The Japanese said: "If you think you hit a victor, she'd still run for it and it feels like you have to restart the point from zero".
The world number nine - in Singapore after an injury to Simona Halep - was a set and a break down to top seed Kerber at one stage, with the Wimbledon champion seemingly set for a comprehensive opening victory.
After trading early breaks, the match settled down as the pair enjoyed more success on serve. "It can only get better (with) every match you play".
Kerber served for the match at 5-4 but Osaka blasted a flurry of winners to break back.
"I just competed really well". "In the third set I had so many chances and I couldn't take it".
Stephens ramped up her aggression in the third set, hitting a crafty drop shot to earn her first match point.
"Obviously playing someone with a lot of confidence is always a tough task, but I think that I played well and I kept a good attitude".