Williams needed nearly 3 hours to best Canadian Aleksandra Wozniak 4-6, 6-4, 7-6 (7-5) as she advanced to the fourth round of her first tournament following a six-month absence.
Williams was sidelined while battling the effects of Sjogren's syndrome -- an auto-immune disease that causes fatigue and joint pain. The three-time Miami champ gained the field this year as a wild card.
She was down to match point in the second set but fought back for the win after defeating Kimiko Date-Krumm and world No. 3 Petra Kvitova in earlier in the event.
Meanwhile, the top-seeded Azarenka of Belarus cruised into the next round with a 6-0, 6-2 decision over Heather Watson in a 79-minute match.