On the Court Philippe Chatrier arena where her compatriot Jannik Sinner had been crowned the men's new world No.1 less than 24 hours before, the diminutive 12th seeded Tuscan Paolini took full advantage of Rybakina's off-day on Wednesday to prevail 6-2 4-6 6-4.
It's the first time an Italian man and woman have reached the semis in the same grand slam.
Kazakhstan's Elena Rybakina produced a performance littered with errors.
"It's an unbelievable feeling, it was a really tough match," beamed Paolini, who's just 1.63m tall but is big on spirit and talent.
The 2022 Wimbledon champion Rybakina, seeking her second grand slam title at Roland Garros, coughed up a remarkable total of 48 unforced errors against the rising Italian.
It felt like a match, at times, that neither woman appeared to want to win, with serve being broken a dozen times during the two-hour three-minute contest in which Rybakina only rally began to fightback for a while in the second set.
"I got a bit too emotional in the second set. But I said to myself 'she's a great champion, it can happen', and I just tried to fight and hit every ball, and I'm here!" smiled Paolini.
"I tried just to stay there in every point and forget what happened in the second set, it happens, it's tennis - it's normal. Accept that and fight again.
"It was my first time playing on this beautiful court, it was a pleasure and a privilege to get my first win here."
Ultimately, the 28-year-old from Bagni di Lucca was rewarded for holding her nerve far better in the denouement, as the Kazakh's game crumbled.
Withe the scores locked at 4-4 in the final set, and the contest feeling too close to call, Rybakina served up a terrible game, hitting first a simple backhand and then an equally straightforward forehand low into the net, before ballooning a forehand long to give Paolini the simplest of gift breaks.
Serving for the match, Paolini then made no mistake, although it somehow felt fitting that it was two final Rybakina errors, with groundstrokes that flew long, which sealed her landmark win.
Paolini, who had accounted for Australian No.1 Daria Saville in her first round last week, was much the brighter from the start, with her Kazakh opponent appearing to be on one of her flat days as she chalked up 16 unforced errors in the first set alone.
It appeared only a matter of time until the favourite found her range but it was only when Paolini broke her for a fourth time to move into a 4-3, 40-15 lead in the second that the seemingly ever-emotionless Rybakina really began to stir, reeling off three games to level the match.
It looked as if the 24-year-old Rybakina had finally got some control when she broke Paolini's serve four times in succession but each time she appeared to gain the upper hand, she kept giving back the initiative to the Italian.
Paolini will next meet either No.2 seed Aryna Sabalenka or 17-year-old Russian star Mirra Andreeva, the 2023 Australian Open junior finalist, in the last-four.
