In the match of her life, the 20-year-old Queenslander produced the best display of her career to prevail 6-3 6-7 (6-8) 6-3, withstanding everything that the player who may be the game's greatest ever could throw at her.
The whole surreal occasion left the quiet, self-effacing youngster amazed. Having never even seen her idol Williams before this week, she laughed after her victory: "When we were in that hallway before we walked out into Centre Court, that was the craziest moment of my life ... just insane.
"It's difficult to explain here aura. I don't know. It just seems like she's just so much better, in general, than you as a person."
But not for one glorious night, at least, as a tennis player.
Joint overcame the disappointment of missing out on a match point when on the verge of a straight-sets win, then battled back after Williams had roared into a third-set lead to seal a monumental triumph in two hours 22 minutes.
The emotional victory left Joint, who grew up in the US state of Michigan where Williams was also born, almost dumbfounded.
"I didn't get much sleep last night, I was up 'til 2am just thinking about it," she told the crowd.
"I've been dreaming about this moment since I was a little kid, so this is pretty crazy."
Williams left her old Centre Court domain as she'd entered it, to thunderous cheers from the crowd, and she offered a wave to all sides. It had a real valedictory feel.
This ultimate competitor, though, had nothing to reproach herself for.Â
For though she may have been more than half-a-step slower and inevitably rusty after nearly four years away from singles action, she was still astonishingly impressive, almost unbelievably so, three months before her 45th birthday against one of the sport's rising stars.
Yet Joint stayed strong, even as seemingly all the crowd were baying in the late evening for a sentimental Serena triumph under the roof, perhaps proving how much the game has moved on in the 23-time slam champion's absence.
Joint had lost 13 of her previous 14 matches stretching back to January and had dropped to No.87 to be only the Australian No.4, but she was still a cut above the old champion, who threw in 190km/h-plus serves and occasional rocket groundstrokes, only to have them, for the large part, returned with interest.
In a way, Joint, who jokily refers to herself as the "Ginger Ninja", looked the perfect target for Superwoman Williams -- a rising star suffering a difficult 'second-season' slump who looked as if she might fold under pressure.
The opposite proved true, though. Yes, Joint did succumb at the end of the second stanza, but she only responded even more fiercely in the decider when the 23-time grand slam champion earned a break for 2-1, moving into her first lead in the final set.
Joint took the next four games as the physical toll really told on the mother-of-two, and there was even a limp by the end of her first singles match since she was beaten by another Australian, Ajla Tomljanovic, at the 2022 US Open.
Williams still has doubles with 46-year-old sister Venus, but, already the oldest player to feature in the women's singles at Wimbledon since 47-year-old Martina Navratilova in 2004, this felt like the end.
Watched by her two daughters and Venus in her player's box, Serena was outplayed by Joint in the opening set, her movement around the court inevitably shown up at times by a player who is 24 years younger.
Her matchless competitive streak was in evidence as she let out a primal scream after saving a break point at 3-4, but Joint moved into a set and 3-1 lead before her work got more ragged under the improving groundstrokes from Williams, who would have beaten plenty of other first-round opponents on this form.
After she had a first match point repelled at 6-5 in the second set tiebreak by a huge serve, and Williams went on to steal the stanza, it looked ominous, but time waits for no athlete and it felt like a career-changer for Joint as she got through to the second round of a grand slam for the third time.
In the next round, Joint will play Alexandra Eala, the Filipino who she beat to win her first grass-court title in Eastbourne last year.
