The No.1-seeded Williams dropped serve at love in the fourth game of the second set but apart from that put on a serving clinic, holding her other six serve games - and breaking the Slovak six times.
"I'm definitely happier with the return game today," Williams said after the match. "I could still use some improvement. She served pretty well, but I was pretty happy with the results from the match today."
Williams has now dropped a total of just 10 games through her first three matches - five against Francesca Schiavone in her opener, three against No.13 seed Kirsten Flipkens in her second match and now two against Rybarikova, who had just won the International-level event in DC last week.
"I'm definitely feeling pretty good, playing much better than I have in the past month, I'd say," Williams said. "I'm glad I'm getting back to the feeling and getting into a good rhythm out there again."
"I just felt like I was too tight, and just put a lot of pressure on myself there," she said of the loss at the All-England Club. "I just needed to relax more than anything, which is what I'm doing now."
In a match where the serve clearly wasn't a huge factor - the players broke each other 10 times in a row at one point in the first set - it was Radwanska who always came through on the important points, including a pair of set points in the first set, one with Errani serving at 5-4 and another with the Italian serving at 6-5. After tucking away the first set Radwanska seemed to go on cruise control, building a 3-1 lead right away, but Errani didn't go away, getting the break back and staying even through 5-all.
But one last push put Radwanska across the finish line after two hours and a minute.
Williams has beaten Radwanska in all five of their previous meetings. Four of those have been routine two-setters, and in the only one that went to three sets - Williams' 61 57 62 win in the Wimbledon final last year - the American actually originally led 61 42 before letting the Pole back into the match.
"It's a good match-up," Williams said. "She does everything so well. She's having a much better year. It's going to be an interesting match. It's definitely not going to be easy. I hope I can play well.
"I can only hope that I come out on top."