She made a slow start in Auckland, with Pegula taking a 3-1 lead in the first set, before recovering to win the next five games and close out the opening set. She defeated Jessica Pegula in the ASB Classic final this past Sunday.
Earlier, the 23-time major victor Serena donated her $43,000 winner's check to the fundraising appeal for victims of Australian wildfires, joining many other tennis stars, such as Ash Barty, Nick Kygrios, Novak Djokovic and Maria Sharapova, who have pledged money to the already months-long fire emergency there.
"It feels good. It's been a long time", Williams said after the match. "It's been a long time; you can see the relief on my face".
In addition to her charitable announcement, Williams' victory on Sunday has yet another reason to celebrate: it was her first title since the birth of her daughter in 2017, who was at play on Sunday.
With the win, Serena moved into history book, becoming the third-oldest woman to win a WTA tournament behind Billie Jean King (39) and Kimiko Date-Krumm (38).
"I've been playing in Australia for over 20 years and it's been really hard for me to watch all the news and everything that has been happening in Australia with all the fire and.animals and people that have lost their homes", she said. "So much has happened, so I decided at the beginning of the tournament, every match I played, I'd donate a dress and I'd also donate all my prize money for a great cause".
Williams eventually achieved a break of her own to level at 3-3, finding the power and precision that had deserted her until then.
Williams smashed two aces and dropped a serve against Pegula.
Pegula took the lead 2-0 before Williams rebounded and scored 3-3 in the first set.
Williams might have had two titles on the same day but she and longtime friend, first-time doubles partner Wozniacki were beaten 6-4, 6-4 by Americans Taylor Townsend and Asia Muhammad in the doubles final.
There were never any doubts, neither for fans, nor for Williams herself, that her physical ability to win a title was still intact.