Kerber had never won a match at the season finale - she went 0-3 last year and was 0-1 this year - but with some phenomenal down-the-line forehands and pinpoint accuracy from just about every other wing, the No.8-seeded German righted all those wrongs, blasting 31 winners to just 11 unforced errors against Radwanska, eventually defeating the No.3-seeded Pole in straight sets in 58 minutes, 62 62.
Her serve was on fire too - she lost 10 points in eight service games and never faced a break point.
"It's not easy to play well against Aga - you need to be focused on every point - so that's what I tried to do," Kerber said. "I was playing very well from the first point to the last and I'm really happy with how I played. I'm feeling very good about my game and I hope I can continue like this tomorrow."
Radwanska finished with an even differential - 10 winners, 10 unforced - but it just wasn't enough.
The standings in the Red Group will now come down to just one match on Friday - Serena Williams has already secured first place as she has gone 3-0, and Kerber and Petra Kvitova will play for second place on Friday as they are both 1-1 now. Radwanska rounds out the group with an 0-3 record.
Kvitova and Kerber are tied in their head-to-head series, 2-2, though Kvitova won their only previous 2013 meeting at Tokyo just a few weeks ago - it was a streaky affair though, Kvitova winning, 62 06 63.
"I will just go out there tomorrow and try to play like I did today and enjoy the tennis," Kerber said. "It feels unbelievable to be here - I've really enjoyed my stay this week and we'll see how it goes.
"The main thing for me is just to try my best and focus on every point tomorrow."
And so Radwanska's season wraps up - three titles at Auckland, Sydney and Seoul, and a sparkling 56-19 record. But within that 56-19 record was a trend that crept in on Friday - she went 53-13 against right-handers this year but just 3-6 against lefties (and all six of those losses came in straight sets).