Jie Zheng of China kisses the trophy after winning her match against Flavia Pennetta in the singles final of the ASB Classic Tennis at the ASB Tennis centre in Auckland on January 8, 2011. Pennetta retired due to injury. AFP PHOTO / Michael BRADLEY (Photo credit should read MICHAEL BRADLEY/AFP/Getty Images)

China's Zheng Jie prepares to return a shot against Italy's Flavia Pennetta on her way to winning their singles final match at the ASB Classic women's tennis tournament in Auckland, New Zealand, Sunday, Jan. 8, 2012. (AP Photo/SNPA, David Rowland)

China's Zheng Jie poses with her trophy after defeating Italy's Flavia Pennetta in their their singles final match at the ASB Classic women's tennis tournament in Auckland, New Zealand, Sunday, Jan. 8, 2012. (AP Photo/SNPA, David Rowland)

China's Zheng Jie holds up her trophy after defeating Italy's Flavia Pennetta in their singles final match at the ASB Classic women's tennis tournament in Auckland, New Zealand, Sunday, Jan. 8, 2012. (AP Photo/SNPA, David Rowland)

China's Zheng Jie (R) goes to shake the umpire's hand as Italy's Flavia Pennetta retires hurt during the singles final of the ASB Classic international tennis tournament in Auckland, January 8, 2012. Reuters photo

China's Zheng Jie won the rain-delayed Auckland Classic on Sunday after fourth seed Flavia Pennetta retired hurt when trailing 2-6 6-3 2-0.

The world number 20 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 an apparent back injury before eventually throwing in the towel.

Italian Pennetta'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.

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 pint-sized opponent roared back into the match by peeling off five straight games.

Zheng, a former semi-finalist at Wimbledon and Melbourne Park, held a nervous service game to clinch the second set and broke Pennetta early in the third before the Italian decided enough was enough.

Once China's highest-ranked player ahead of French Open champion Li Na, Sichuan native Zheng's ranking has dipped to 48th after a disappointing 2011. But she has shown strong form in the leadup to the year's first grand slam, upsetting former world number two Svetlana Kuznetsova in the Auckland semi-final.

(Agencies)