Ashleigh Barty faces her first seeded challenge of the US Open after Greek star Maria Sakkari lived up to growing expectations to book a third-round date with the world No.2.

Enjoying a career-high ranking inside the top 30 after snaring her maiden WTA title in 2019, Sakkari ousted Chinese qualifier Peng Shuai 6-7 (5-7) 6-4 6-2 to continue her breakout season.

Barty, a second-round winner over American Lauren Davis on Wednesday night, is the last Australian woman standing for the sixth straight major following Ajla Tomljanovic's exit.

Tomljanovic fought hard in searing heat before bowing out 4-6 7-5 6-2 to Estonia's 21st seed Anett Kontaveit on Thursday.

Barty was looking to raise her game after needing more than two hours to see off the 73rd-ranked Davis and three sets to beat world No.80 Zarina Diyas in her opener.

But having finished second best against Barty twice already this season, the second time as recently as this month in Cincinnati, Sakkari knows she's in for a fight regardless.

"I said last year in Indian Wells when I beat her, for me she is the best player in the WTA Tour," Sakkari said.

"And that was when she was like 14, 15 in the world. You could see how talented she is, how mentally strong she is and how calm.

"You could see was going to achieve many good things and we can see how she has played this year.

"But I also believe in myself and try my best tomorrow for the best result for myself.

"I remember I took my foot off the gas in 'Cinci', after I won the first set. So if things go well tomorrow, I will keep it in mind not to do the same mistake."

The winner on Friday will play either Frenchwoman Fiona Ferro or China's 18th seed Qiang Wang for a place in the quarter-finals, where Serena Williams could be lying in wait.

Barty has yet to rediscover the golden touch that propelled her to world No.1 during the European clay and hardcourt season.

But the 23-year-old is rising in confidence, after following up a loss to Sofia Kenin in her opening match of the American hardcourt swing with a run to the semi-finals in Cincinnati.

"That's half the battle sometimes, is to win when you're not playing your best tennis," Barty said.

"Cincinnati we had to fight really throughout that whole event to stay alive. There were some scrappy matches. To make a semis is really good.

"Obviously the matches that were after - not the semi-final (loss to Svetlana Kuznetsova) but more the matches and the conditions - were about getting used to being in match situations again.

"Because nothing on the practice court can give you that. So we're really pleased with that.

"Obviously it's paying dividends so far this week. To be in another third round is really, really positive."

AAP