Bianca Andreescu had thought herself a champion many times. Now it's true.

The 18-year-old Canadian, who practices visualisation every morning, upset three-time major champion Angelique Kerber 6-4 3-6 6-4 to win the WTA title at Indian Wells.

She became the first wildward winner of the WTA Premier Mandatory event - one of the four biggest tournaments outside of the grand slams and the season-ending WTA Finals - and second-youngest to claim the event.

"The fricking champion of Indian Wells," Andreescu said. "It's crazy."

She overcame nerves, fatigue, arm and leg issues and fatigue in the final set to earn the maiden title of her fledgling career.

Andreescu won on her fourth match point when Kerber netted a backhand.

She broke the German three times in the third set, rallying from a 3-2 deficit to take four of the final five games.

"This moment has become a reality so it's really, really crazy," Andreescu told the crowd before speaking some Romanian.

Born in Canada, she later moved with her parents to Romania, where she first started playing tennis.

Kerber was the last of five seeded players - and fourth top-20 player - Andreescu knocked off in her seven matches in California.

"When she had the chances, she just go for it," Kerber said.

Andreescu said, joking.

Andreescu is projected to rise 36 spots to No.24 in the WTA Tour rankings on Monday after she started the year at 152nd.

World No.8 Kerber remains without a title since she won Wimbledon last year.

Leading 2-1 in the third, Andreescu took a medical timeout and had a trainer massage her tight right shoulder and arm. She dropped the next two games but bounced back to grab a 5-3 advantage.

"I just fought till the end because physically I wasn't feeling too well," she said.

The trainer reappeared again to ice Andreescu's cramping legs.

Andreescu squandered three match points on her serve but couldn't close out the match.

However, she broke Kerber's serve one final time to secure the biggest win of her career.

"At the end I was not able to take my chances, but she did," Kerber said.