Serena Williams ended a three-year title drought and donated her winner's cheque to victims of the Australian bushfires in an emotional WTA Auckland Classic final on Sunday.

Williams raised expectations for this month's Australian Open, where she can equal Margaret Court's record of 24 Grand Slam titles, with her 6-3, 6-4 victory — her first WTA trophy since she won in Melbourne in 2017.

But the 38-year-old tempered celebrations as she donated her US$43,000 winners cheque to the Australian bushfire relief fund and described how the tragedy had affected her deeply.

“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.

“I decided at the beginning of the tournament [...] I'd donate all my prize money for a great cause.”

It is Williams' first title since 2017 — and her first as a mother — since she won the Australian Open while pregnant. Her 73 WTA titles now stretch across four decades, after she won her first in 1999.

The tournament top seed slipped 1-3 behind in the first set against the unseeded Pegula, a fellow American.

But once she found her range there was never any doubt about the final result, which Williams greeted by raising her arms in triumph while her husband Alexis Ohanian and two-year-old daughter Olympia looked on.

“It's been a long time, I think you could see the relief on my face,” she said, adding she could feel her game sharpening up as she prepares to head to Melbourne.

“It definitely feels good, it feels like i was definitely improving as the week went on and obviously I needed to.”

Yelling with every point

Pegula, who has only one title to her credit, had stunned former world number one Caroline Wozniacki — a close friend of Williams — in a three-set semi-final, winning every game in the deciding set.

The 25-year-old continued in the same fearless vein at the start of the final, seemingly untroubled by her heavily bandaged left thigh as she chased down everything Williams delivered and even broke Williams' first serve.

Pegula held her own serve and appeared set to break again when Williams, by this stage yelling with every point she won, fought back from 15-40 to hold her second serve on the fifth deuce.

Williams eventually achieved a break of her own to level at 3-3, finding the power and precision that had deserted her until then.

With her confidence boosted, Williams held to love in the next game, broke Pegula again and then served to clinch the first set.

Pegula was down 0-40 at the start of the second set before rallying to hold serve but the strain of facing the player who has dominated women's tennis for two decades was showing.

Williams broke on Pegula's next service game and stayed in front until the end of the set to take the title and end a sequence of five defeats in finals since her 2017 win in Melbourne.