Ranked a career-high No.28 in February, the Australian's stocks have dropped steadily throughout the year and she entered this WTA 500 event ranked 94th.
However, the American-born Aussie showed the numbers can't always be trusted as she powered past sixth seed and world No.16 Sorana Cirstea of Romania.
Joint's impressive 6-3 7-6 (7-4) victory on Tuesday secured her a spot in the round of 16.
It also continued her recent resurgence, after she made the final at last month's Korea Open in Seoul, where compatriot Kim Birrell claimed her first WTA title with a 6-4 6-2 win in the final.
Joint's next opponent is Spanish world No.42 Cristina Bucsa, who beat France's Diane Parry 7-5 6-1 on Tuesday.
And Joint isn't the only in-form Australian still in the main draw.
On Monday, Talia Gibson, who recently reached the Singapore final, came through an all-Australian affair over qualifier Daria Kasatkina 6-0 7-5.
Gibson powered through the first eight games in a row but was made to work hard to hold off the former world No.8 in the second set.
Her reward is a round-of-16 clash with eight-seeded Aleandra Eala, of the Philippines.
Eala won the battle of former US Open junior champions, easing past 17-year-old British wildcard Mika Stojsavljevic 6-1 6-2 in just 75 minutes.
The popular Philippines player claimed the girls' title at Flushing Meadows in 2022, two years before Stojsavljevic lifted the trophy.
The Tokyo event has been hit hard by injuries, with two of the tournament's biggest names withdrawing.
Poland's Maja Chwalinska, the No.2 seed, and Switzerland's Belinda Benic, seeded fourth, both pulled out before the tournament kicked off.
Chwalinska, who won her first career WTA title in Wuhan two weeks ago, pulled out with a left thigh injury.
And injury also brought an early end to the season for Bencic, who suffered partial tears of the tibiofibular (syndesmotic) ligament in her right foot.
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