The last time the two played, in the third round of Miami last March, Williams had to battle more than two hours to prevail, and this one looked like it was going in the same direction as Dellacqua hung tough with the big-hitting seven-time Grand Slam champion to build an early 6-3, 1-0 advantage.
The No.7-seeded Williams flipped the script completely from there, though, reeling off 12 of the next 14 games to completely run away with it after an hour and 34 minutes on the court, 3-6, 6-2, 6-1.
Williams is now 13-2 on the year, a record that includes her 46th WTA title in Auckland.
"I've been going non-stop this year actually," Williams said in her post-match press conference.
"I've played so much, and it's only February. But so far I'm happy with how it's going."
Next up for Williams will be Barbora Zahlavova Strycova, who was two points from losing the first set serving at 4-5, 0-30 and trailed 3-1 in the second set but managed to beat Sabine Lisicki, 7-5, 6-4.
Williams has beaten Zahlavova Strycova in both of their previous meetings, but their last meeting was an absolute thriller at Charleston last April, with a very special name watching from the stands.
"I played her about a year ago and she's had an amazing last 12 months, I think some of the best of her career," Williams said of the Czech veteran. "I'm sure she feels confident. I'm not really sure what to expect. I don't know if her game has changed, so I have to be ready for whatever happens."
The only other seed in action wasn't so lucky, with No.8 seed Angelique Kerber falling to Victoria Azarenka, 6-0, 6-3. Azarenka is now on an 11-match winning streak at the Qatar Total Open, having captured the tournament in each of her last two appearances, in 2012 and 2013 (read more here).
The only other day session match saw Flavia Pennetta come back to beat Svetlana Kuznetsova, 3-6, 6-4, 6-2. Kuznetsova led 6-3, 3-2 and held triple break point in that game before Pennetta stormed back.
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