RIYADH, Nov 9 (Reuters) - Coco Gauff beat China's Zheng Qinwen 3-6 6-4 7-6(2) in three hours to the become the youngest player to win the WTA Finals since Maria Sharapova in 2004 and the first American champion since Serena Williams 10 years ago.

The 20-year-old Gauff stormed to a 6-0 lead in the final set tiebreak for six championship points as she eased to victory.

"It was a really great match. Qinwen played some unbelievable tennis," third seed Gauff said. "I was just trying my best to hang in there and I never gave up.

"I was just telling myself, 'it's another point, another chance'. I've been in situations like this in the past and have been able to turn it around ... and was able to do it again today."

Olympic gold medalist Zheng, seeded seventh and making her WTA Finals debut, won the first set in just under an hour, saving all five break points she faced to keep Gauff at bay. The American came back from 3-1 down to take the second set.

(Reporting by Shifa Jahan in Bengaluru; Editing by Ken Ferris)

((Shifa.Jahan@thomsonreuters.com;))