The 19-year-old overcame a mid-match wobble to put paid to 18th-seeded Brazilian Beatriz Haddad Maia 6-4 4-6 6-4 on Saturday (local time).
Joint proved much more effective on her first serve, landing 70 per cent of her first delivery compared with just 55 per cent for the Brazilian.
In a match that featured a swag of break-point opportunities, Joint had a slight advantage, converting seven of the 19 chances she set up, compared with six of 17 for Haddad Maia.
It was a superb win against the Brazilian, who is certainly no slouch, having won four tournaments in a career that has taken her to a highest ranking of No.10 in the world.
Victory for Joint on the Cincinnati hard courts set up a third-round meeting with 12-seeded Russian Ekaterina Alexandrova.
Joint reached a career-high singles ranking of No.37 last month and is currently Australia's No.2 behind Daria Kasatkina.
Looking to join the 19-year-old in the third round in Cincinnati are fellow Aussies Kasatkina, Kimberly Birrell and Ajla Tomljanovic, who play their second-round matches on Sunday (local time).
Kasatkina, seeded 15th in Cincinnati, is favoured to advance when she takes on Italy's Lucia Bronzetti.
Birrell faces a daunting challenge, up against American fourth seed Jessica Pegula, who reached the final of last year's US Open.
Tomljanovic will also have her hands full when she takes on Denmark's Clara Tauson, who beat major winners Iga Swiatek and Madison Keys on her way to the semi-finals of last week's Canadian Open.
Swiatek bounced back from her shock loss to Tauson in Montreal by beating Russia's Anastasia Potapova 6-1 6-4 in the second round in Cincinnati on Saturday.
The Polish superstar, winner of six major singles titles, powered through the first set, then had to fend off four break points in the second.
She next plays Ukraine's Marta Kostyuk, who she has defeated in all three previous meetings, in the third round.
Also through to the third round is British No.1 and 2021 US Open champion Emma Raducanu, who beat Serbia's Olga Danilovic 6-3 6-2.
Australian Open winner Keys had a tougher start to her Cincinnati campaign, saving two match points to survive against German Eva Lys with a 1-6 6-3 7-6 (7-1) win to punch her ticket to the third round, where she will play Japan's Aoi Ito.
"I just kept trying to listen to everyone here cheering me," said American Keys, who triumphed in 2019 and clinched Saturday's affair with an unreturnable forehand. "It helped me get back into the match and across the finish line."
The Cincinnati Open is a vital lead-up to the year's final major, the US Open, played on the hard courts at Flushing Meadows in New York from August 24.
- with Agencies
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