Williams is 6-0 against Radwanska, including the Wimbledon final last year.
''It was definitely one of the tougher matches,'' Williams said. ''Obviously, Wimbledon was another tough one. We actually have a lot of good matches.''
The 27th-ranked Cirstea ousted two former world No. 1s in Jelena Jankovic and Caroline Wozniacki this week and beat sixth-seeded Petra Kvitova in the quarterfinals.
''A new day, a new match,'' Cirstea said. ''It doesn't matter who I beat the day before or what I've done. It's just another day where I have to get out there and do the things to show the work that I've put in.''
Williams has seven tournament victories this year and 53 overall. In her last event, she won the Swedish Open on July 21. She has played Cirstea twice, handily winning both matches.
''She's definitely not an easy player to play,'' Williams said about Cirstea. ''Her results recently have been really consistent and she's found herself and she's playing better and better and more confident. It's going to be a really tough match.''
Cirstea won her lone WTA Tour in 2008 at Tashkent.
''The main thing is I have to serve well, be aggressive, try to be in control, and go into the court with good mentality,'' Cirstea said about the final.
Radwanska and Williams traded breaks twice in the first set, with the Pole keeping up with Williams's imposing power game. Radwanska dashed corner-to-corner at the baseline to make returns and led 6-5, but Williams held serve to force a tiebreaker. In the tiebreaker, Williams blasted a cross-court winner for a 6-3 advantage, then ended the set with an ace.
Williams called for a medical timeout early in the second set because of ''gastrointestinal issues.''
''I had some stomach issues and I had to settle them down,'' Williams said. ''I'll be fine. I feel good.''
She looked slightly out of sorts early in the set, going down a break and screaming at the ground after blowing an easy overhead smash.
But that didn't last.
Williams broke back and took a 4-3 lead by running Radwanska around the court with blistering offensive groundstrokes. She won a nearly 10-minute game to pull ahead 5-4 and broke Radwanska to finish off the match in 1 hour, 52 minutes.
''She's was really playing a great match today,'' Radwanska said. ''This is what makes her No. 1 in the world — playing unbelievable in the very important moments and very powerful shots. I couldn't do anything about that. I had some break points and I didn't take them.''