Radwanska was on her game - she had a +13 differential during the match, 23 winners to 10 unforced errors. But Keys was lethal from every wing, blasting aces and ripping winners off both forehands and backhands, hitting 67 winners in all. The American even had a better differential of +16 (67 to 51).
But this time experience prevailed over youth, and the No.4-seeded Radwanska grabbed the only break of the third set - though she needed 10 chances to get it - and she ended up a 75 46 63 winner.
"She was playing great tennis," Radwanska said. "In the second set two easy mistakes cost me one set. So in the third set I had to really focus on my serve and take my chance when I got it. She didn't put so many first serves in during that game, and there was my chance, and that's why I won the set.
"She was serving unbelievable. When I had break points, a couple of times I couldn't do anything."
For the first time in a while, Radwanska isn't the only Polish player into the second week of a Grand Slam - Jerzy Janowicz and Lukasz Kubot have both made the cut on the men's side of the draw.
"I'm just very happy to see much more Polish names in the draw," Radwanska commented. "Of course, Jerzy and Lukasz are playing really great tennis. I'm happy for them, because they're working really hard and they deserve it. Polish tennis is getting a bit bigger, and I'm really happy about that."
And what about Keys? "A couple of months ago some journalist asked me which one of the young players was going to do some good results, and I picked her. I think she's a very good up-and-coming player. She can do really well. If she works hard and plays like this, we will see her much more often."
Keys had already beaten a Top 5 player - she beat a No.5-ranked Li Na earlier this year - but she came close to her first win over a Top 4 player. "After the first set it was so close and could have gone either way," she said. "I was realizing I can do this and stay in it and fight. I did a good job getting back in the second set and staying close in the third, so I'm happy with the match today, pretty much."
It has been a very successful year so far for Keys - she started the year ranked No.137 in the world and after this tournament she will break the Top 50. "It's just gotten better and better," Keys said. "There has been a lot of improvement. The setbacks come and you work through them, and you work forward. I'm just really excited for the US Open Series and the US Open. Hopefully I can keep it up."
Next up for Radwanska is Tsvetana Pironkova, who regrouped after dropping the first set to beat Petra Martic in three sets, 61 46 62. Pironkova has had by far her best Grand Slam results at this Grand Slam - she is now 14-7 lifetime at this event but just 16-23 at the other three Grand Slams combined.
Radwanska leads Pironkova in their head-to-head series, 7-2, though there is one dangerous sign in their history - Pironkova actually won their last grass court meeting, at Eastbourne a year ago.