The 38-year-old American succeeded in her mission to land a first trophy since landing her 23rd grand slam at Melbourne Park in 2017.
She was absent from the tour for 12 months while becoming a mother and had since been unable to capture a title, losing five finals — including four in grand slams.
The defeats were becoming a worrying trend, but a 6-3 6-4 victory over compatriot Jessica Pegula in the Auckland Classic final on Sunday broke that streak.
Now Williams heads to Australia with a 24th slam her target as she bids to move level with Margaret Court at the top of the all-time list.
"It's pretty satisfying just to get a win in the finals," Williams said. "I think that was really important for me and I just want to build on it.
"Obviously, it's just a step towards the next goal."
Williams could not make it a twin success as she and Caroline Wozniacki fell to Asia Muhammad and Taylor Townsend in the doubles final.
"I'm OK with what happened," Williams said.
"I'm glad I was able to get a title in the singles — it's been a while. But I've been in the finals a lot, so I just keep going and just keep trying."
Victory over Pegula gave Williams a 73rd WTA singles title, meaning she has won tournaments in four decades, starting with five she won in 1999 as a teenager.
The Australian Open begins on Monday, 20 January.