Top-ranked Ash Barty's new year started with an upset loss to American qualifier Jennifer Brady at the Brisbane International.

The 53rd-ranked Brady, who has no WTA tournament titles, beat the French Open champion on her home court at Pat Rafter Arena 6-4, 7-6 (4) on Thursday.

It's been quite a week already for Brady, who went through three rounds of qualifying and then wild-card entry Maria Sharapova in the first round. Barty was coming off a first-round bye.

In later matches, defending Australian Open champion Naomi Osaka beat Sofia Kenin 6-7 (3), 6-3, 6-1 and two-time Wimbledon champion Petra Kvitova defeated Ludmilla Samsonova 6-3, 6-2.

For the 24-year-old Brady, it's the first time in three matches she has defeated Barty, who remains in contention in the doubles.

Brady's baseline game was crucial to the win, swinging hard throughout and never letting a rusty Barty break her rhythm. She had a practice hit with Barty earlier in the week, and drew confidence from it. Big back-to-back wins are evidence of that.

“I think just going out there and knowing that I can win, playing to win instead of just playing to just hang in there or try and just hold my own,” Brady said. "I think just believing in myself, having the confidence in the game, knowing that I am a player who is able to dictate.

“And if I'm able to do that and giving myself the best opportunity at winning — and win or lose knowing that I played within myself and believed in myself — I think I'll walk off the court pretty happy.”

Barty, who won the season-ending WTA Finals last season, has failed to get past the second round in five appearances at the Brisbane International. The Australian's lack of match play could be seen in her 26 unforced errors but she was also eager to pay credit to Brady, who lost only one point on her first serve and didn't have to face a single break point.

"You can call it rust or you can just call it a bit of execution," Barty said. “For me it was on those returning games. I had probably had half a dozen second-serve returns where I failed to make it into the court, in that first set in particular. And then Jen grew in confidence. You have to give credit where credit's due.”

The 23-year-old Barty will continue playing doubles in Brisbane and is in the field for next week's Adelaide International before starting her Australian Open campaign.

“It's rare in sport that you get an opportunity every single week and so quickly,” Barty said. "That's one of the beautiful things about tennis is that next week you get a chance in Adelaide to kind of come out there and try and execute that little bit better."