The No.14-seeded Radwanska was the first of the star pair to fall, showing a mighty fightback in the second set but ultimately succumbing to the rock solid ground game of Beck in the third, 6-2, 3-6, 6-1.
It was Radwanska's first time losing in the first round of a Grand Slam since all the way back at the 2009 Australian Open - but there might be better days just around the corner, just across the water.
"I was doing a lot of things to play well here," she said. "It doesn't matter how I feel, I should win that match. I'm not happy about the result. Losing first round at a Grand Slam is always disappointing.
"But clay season is not really my favorite one, and I guess it's not getting better, so I just want to forget about that and focus on grass, I guess. I will do everything in my power to play better there."
The No.15-seeded Williams followed Radwanska to the sidelines later in the day, looking strong early on - even holding a 3-1 lead in the first set tie-break - but going quietly to Stephens, 7-6(5), 6-1.
Perhaps surprisingly, it was the first time Williams and Stephens had ever played each other.
"I didn't really know what to expect," Stephens said. "I mean, obviously she's a great player, and a great champion. I knew I was just going to have to get out there and do my best. Obviously I've never played her in a real match, so I didn't know if she was going to boss me off the court, or how that was going to go. But I knew if I just stayed in there and played my game that it would be a good match."
"My opponent played well," Williams said. "Sloane had a great week in Strasbourg and this was a tough first round match. When you arrive at any tournament you hope you will have a chance to work your way into the event but I didn't have that opportunity - she just played better than me today.
"I had some good practices coming in but things don't always work out the way you would like. I have a little while now between tournaments and so now I'll just get ready for the grass at Wimbledon."
The other top seeds in action all moved through in straight sets, with No.2 seed Maria Sharapova holding off the always-dangerous Kaia Kanepi, 6-2, 6-4, No.8 seed Carla Suárez Navarro storming past the crafty Monica Niculescu, 6-2, 6-2, No.11 seed Angelique Kerber routing Timea Babos, 6-0, 6-1, and No.12-seeded WTA Rising Star Karolina Pliskova winning a tight one over Zhang Shuai, 7-6(4), 6-4.
Other winners as the first round continued were No.17 seed Sara Errani, No.19 seed Elina Svitolina, No.20 seed Sabine Lisicki, No.26 seed Samantha Stosur, No.27 seed Victoria Azarenka, No.29 seed Alizé Cornet and No.30 seed Irina-Camelia Begu. First round action in Paris wraps up Tuesday.