Even though she had won all five of the pair's previous meetings, Williams was definitely pressed from the very beginning - Radwanska wasn't just doing her best impersonation of a wall, getting as much as she could back into the court, but she was also bringing out the kind of variety she has become known for, throwing Williams off with different spins, a variety of trajectories and changes of direction.
Twice Williams went up a break in the first set and both times Radwanska broke right back, and handily too, at 15 with Williams serving 3-2 and at love with her serving 5-4. But Williams would get the last word in in the first set, blasting a huge ace up the middle to tuck away the set in a tie-break.
Williams struggled early in the second set, falling behind 3-1, but she caught up quickly and would eventually hit back-to-back winners to close it out, a huge backhand winner down the line and then a forehand volley into the open court on match point. The No.1 seed edged the No.3 seed, 76(3) 64.
"I hadn't played her in a little while and kind of forgot how flat she hits and how low she hits," Williams said afterwards. "But it was okay. Once I accepted that the balls were going to be flat, then it was a little better. It was definitely one of our tougher matches - we actually have a lot of good matches."
The numbers told a clear story of aggressor and defender - Williams had 42 winners to 46 unforced, Radwanska 8 winners to 24 unforced. But there was a subplot within the differentials, as one would think Williams' -4 against Radwanska's -16 would spell a lopsided result, which clearly it was not.
"This was definitely a much better match than, for example, in Miami," Radwanska said. "It was really close and I had my chances but wasn't really taking them. She definitely played well, especially in the tie-break. And she played much better in the end of the second set, playing aggressive and deep.
"That's what makes her No.1 in the world, playing unbelievable in the very important moments, and just hitting some very powerful shots. I couldn't really do anything about that against her tonight."
Williams will now go for her 54th WTA title - she has 53, which ties her for ninth place on the all-time list of WTA titles with Monica Seles, and getting her 54th will make her the standalone No.9 on the list.
Williams has beaten Cirstea in both previous meetings, and handily in straight sets both times.
"Sorana's a great player," Williams said. "She plays really tough. Gosh, she hits really hard. She has a really big serve. She moves well. She's definitely not an easy player to play, but she's - wow.
"Last few months she has been really consistent. She's just a player who has finally found herself, and she's playing better and better and getting more confident. It's going to be a really tough match."