Kuznetsova said her thick hair had been hitting her eye whenever she moved to hit a forehand.
Speaking at a post-match press conference in Singapore, Kuznetsova said her thick hair, which was put into a long braid, had been hitting her eye whenever she moved to hit a forehand.
"I thought, 'Okay, what's more important now, my hair which I can let grow or the match?'," she said, adding that she decided to "go for it".
Shortly after the third set started, Kuznetsova, who was emotional at moments, asked the umpire for "large scissors" during a break and started hacking at her braid.
"I was like just cutting, I was like, I cannot cut it because it's so thick and wet... I don't even now how much I cut there," said Kuznetsova, who has yet to examine her handiwork.
Two games after the haircut, Kuznetsova was also seen sobbing into her towel during a side change.
Asked about it, she said: "I tried to behave and act like I'm professional athlete, professional player. I mean, sometimes it's not easy, you know."
The Russian edged into the competition late Saturday and sent Britain's Johanna Konta packing. She eventually beat defending champion Radwanska 7-5 1-6 7-5.
The oldest player in the tournament, Kuznetsova has battled exhaustion after a long season, and jetted straight to Singapore after lifting the Kremlin Cup in Moscow.
Last November, Andy Murray also gave his fringe a trim courtside at the ATP World Tour finals, saying it was getting into his eyes.