Williams, who lost as many as four games in a set once while going 3-0 in the Green Group, advances to Saturday's semifinals against the White Group runner-up. Williams collected her third WTA Championships title last year without dropping a set.
Kvitova, the 2011 Championships winner and fifth seed this year, had three break chances in the first game of the match but only managed one more the rest of the way. She didn't finish off any of those opportunities while Williams when 3-for-6 in break conversions.
Kvitova, 1-1 in the group, will go against eighth-seeded Angelique Kerber Friday to decide who advances from the Green Group.
Kerber improved to 1-1 in round-robin play with a 6-2, 6-2 romp over third-seeded Agnieszka Radwanska in 58 minutes. Kerber dropped only 10 points in eight games serving and didn't face a break point. She converted four of her five break opportunities in dropping Radwanska to 0-3 in the group.
In Thursday's White Group match No. 4-seeded Li Na got past seventh-seeded Jelena Jankovic 6-3, 2-6, 6-3, leaving Li (2-0) as the only unbeaten player in the group. Li piled up 21 break chances versus Jankovic, finishing off six of them. Half of the breaks came in the final set.
Li plays world No. 2 Victoria Azarenka (1-1) Friday while Jankovic (1-1) meets sixth-seeded Sara Errani (0-2). Errani has been eliminated from advancing but any of the three other White Group players could advance to the semifinals.
Swim Week Miami Beach 2014 [PHOTOS]