Fernandez, a former US Open finalist now ranked No.24 in the world, collected the biggest title of her career on Sunday when she won the DC Open in Washington, thrashing Anna Kalinskaya 6-1 6-2 in the final.
But her return home in triumph did not last long, Joint beating the Quebec native 6-4 6-1 in Montreal on Tuesday (local time).
It was quick revenge for Joint, who lost 6-3 6-3 to Fernandez in the first round in Washington.
Fernandez later criticised the scheduling in Montreal, saying she had been hoping for extra rest before returning to the court in Canada.
She said she had received "a lot of promises" that she would play during Tuesday's night session, giving her more time to recover between the two tournaments, but learned on Sunday while travelling that wouldn't be the case.
"That hurt me because I was very looking forward to be playing at night, but I guess it's a little bit political issues at that point," Fernandez said.
Instead, top-seeded Gauff played fellow American Danielle Collins to open the night session.
Montreal tournament director Valerie Tetreault said she promised Fernandez she would "fight so that she could have the time she wanted", but that ultimately the WTA Tour determined scheduling.
The scheduling almost backfired, with Gauff being pushed to the brink before squeaking past Collins 7-5 4-6 7-6 (7-2).
On a much happier note, Joint, ranked 45th in the world and already a two-time WTA title winner this year, now meets American 28th seed McCartney Kessler.
If she wins that match, next in line would be rising Russian Mirra Andreeva, who is yet to play in Montreal having received a bye, then a walkover after Bianca Andreescu withdrew.
That would be in the round of 32, a stage another Aussie has already reached. Seeded 15, Daria Kasatkina was given a bye in the first round, and on Tuesday she beat Anna Blinkova 6-1 6-4.
Kasatkina next faces 24th-seed Marta Kostyuk, of Ukraine.
Elsewhere, No.7 seed Jasmine Paoloini was surprised by Japan's Aoi Ito 2-6 7-5 7-6 (7-5), but eighth seed Emma Navarro progressed, as did ninth-seeded Elena Rybakina.
- with AP
