Williams is 2-0 in the Green Group after a 6-2, 6-4 decision over Radwanska, who was virtually eliminated after losses each of the first two days of the tournament.
Williams improved to 8-0 lifetime against Radwanska, a string of matches in which she dropped only one set. Williams fought off seven of the eight break points she faced while converting four of the five she had herself.
Azarenka slipped to 1-1 in the White Group when she was beaten 6-4, 6-3 by seventh-seeded Jelena Jankovic. Azarenka was on the defensive much of the day facing six -- and losing two -- break points in the first set and Jankovic had 11 break chances in the second set.
Jankovic is tied with fourth-seeded Li Na atop of the White Group at 1-0. They play each other in Thursday's first match.
Li defeated No. 5-seeded Sara Errani 6-3, 7-6 (7-5) Wednesday. Errani, 0-2 in the group, lost to Azarenka Tuesday. Li claimed the second-set tiebreaker after both players had three breaks in the second set.
Williams will look to complete her group round-robin sweep in the final match Thursday when she takes on No. 5-seeded Petra Kvitova (1-0). Williams is 4-0 all-time versus Kvitova. Also in the Green Group Thursday Radwanska goes against eighth-seeded Angelique Kerber (0-1).
Costly malfunction causes beer flood at Boston-area brewery
Video of Victoria’s Secret models trying to 'twerk' hits Instagram