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.
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 a two hour 22-minute classic.
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 has such an aura, she's such a legend," 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, which had a real valedictory feel.
This ultimate competitor 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 her US-born opponent stayed strong, even as seemingly all the crowd were baying 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, a shy, self-effacing figure who jokily refers to herself as the "Ginger Ninja", looked the perfect opponent for 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 began to tell on the mother-of-two, and there was even the sign of a limp at the end of her first singles match since she was beaten by another Australian, Ajla Tomljanovic, at the 2022 US Open.
Williams still has the doubles to play with her 46-year-old sister Venus, but, already the oldest player to feature in the women's singles at Wimbledon since a 47-year-old Martina Navratilova in 2004, it's hard to imagine this was anything but the end of her singles career.
Watched by her two daughters and Venus in her player's box, Serena was outplayed by Joint in the opening set, defying the suggestions that the occasion might prove too big for her as her ball-striking proved every bit as crisp as the great one, even if she couldn't cope with some of the monstrous serves.
But her movement around the court was 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 was dominant, moving into a set and 3-1 lead before her work got more ragged under the improving Williams groundstrokes.
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.
