China's Zheng Jie won the rain-delayed Auckland Classic on Sunday after fourth seed Flavia Pennetta retired hurt with a back injury when trailing 2-6 6-3 2-0.
World number 20 Pennetta had a medical time-out at the change of ends when leading 5-2 in the first set and called for the physiotherapist several more times in the match to work on her lower back before eventually throwing in the towel.
The Italian's withdrawal gifted 28-year-old Zheng her fourth WTA title and her first since 2006, while providing a welcome boost ahead of next week's Australian Open.
"I am so excited and so happy to win this title because this is the first time I've won my first tournament of the year," Zheng said.
"I want to say sorry for Flavia and the problems with her back. I hope she is better for next week."
Persistent showers washed out the final on Saturday and continued on Sunday, forcing the match to be moved indoors to a smaller stadium where Pennetta, Italy's first top-10 player, quickly seized the initiative.
The 29-year-old broke Zheng twice to seal the first set and raced to a 2-0 lead before her diminutive opponent roared back by peeling off five consecutive games.
Zheng, a former semi-finalist at Wimbledon and Melbourne Park, held a nervy service game to clinch the second set and broke Pennetta early in the third before the Italian decided enough was enough.
Pennetta, who made the quarter-finals of the U.S. Open last year, was scheduled to play this week's Sydney International for another warm-up before Melbourne Park but had withdrawn from the singles due to the injury, tournament organisers said.
Sichuan native Zheng was once China's highest-ranked player ahead of French Open champion Li Na but her ranking has dipped to 48th after a disappointing 2011.
She has shown strong form in the leadup to the year's first grand slam, however, upsetting former world number two Svetlana Kuznetsova in the Auckland semi-final.