In a clash worthy of a final between the current and former world number one, Swiatek showed the steely grit she possesses in abundance but has rarely needed at her favourite major, to extend her streak to 16 wins after triumphs in 2022 and 2023.
"For sure this match was really intense. Much more intense for a second round than I ever expected," Swiatek said.
"Naomi played amazing tennis with loose hands. Sometimes risking a lot but playing in. She played really great. I'm happy she's back and playing well."
Swiatek's clay prowess makes her a firm favourite against anyone but the top seed, who entered the contest on the back of 13 wins and titles in Madrid and Rome, faced huge pressure and was a point away from dropping the first set.
In testing conditions under the roof on Philippe Chatrier, the powerful Osaka posed a litany of problems in one of her best matches since coming back this year from a maternity break, but did not capitalise while up 40-30 at 5-4, hitting a shot long.
Three-times champion Swiatek raised her level and ran away with the opening set in the tiebreak but the Pole found herself trailing 4-0 in the next set as Osaka shrugged off any disappointment to comfortably level the contest.
Osaka saved two break points to hold at the start of the decider and fired a sublime backhand crosscourt winner in the next game to pounce for a 2-0 lead, which the Japanese player extended on serve after another almighty battle.
A rattled Swiatek clawed her way back from 5-2 down and saved a match point before taking full advantage of late errors from a nervy Osaka's racket to complete a famous victory and avoid her earliest exit from the Grand Slam.
Defeat was a bitter blow for Osaka, who produced one of her most memorable performances on her least favourite surface after fully dedicating herself to it in the build-up to the major and left the main showcourt to a standing ovation from the crowd.
"Maybe she's going be a claycourt specialist in a while," Swiatek said.
French fans under fire
The Pole, however, had a message for the French Open fans, known to be a fickle crowd.
"Basically for you, because it's entertainment and we make a lot of money thanks to you, sometimes when we are under a lot of pressure you scream during the rallies and it's hard to stay focused," Swiatek told the crowd.
Her intervention was met with cheers, and only a few boos.
"This is serious, we're fighting to be better and better, there's a lot of money to earn here. You can support us between the rallies but not during the rallies, please.
"I know the French crowd gets players they don't like and they boo, but I know you love me so let's keep that."
It was not the first time that the Roland Garros faithful, known to show up late for the afternoon matches on the back of long lunches and sometimes pick on players, were in the spotlight for the wrong reasons on Wednesday.
Belgian David Goffin accused partisan fans of "total disrespect" and said he had been spat at by a spectator during his marathon first-round victory over local favourite Giovanni Mpetshi Perricard.
"It's really too much. It's becoming football, soon there will be smoke bombs, hooligans and fights in the stands. It's starting to become ridiculous. Some people are there more to cause trouble than to create an atmosphere," Goffin said.
Tournament organisers said officials would ensure that fans followed the rules and respected players.
"The public are incredibly enthusiastic, particularly on the outside courts," they said in a statement on Wednesday.
"However, they must of course show full respect to all players while doing so."
Only nine matches were expected to be completed on Wednesday with Carlos Alcaraz and former runners-up Stefanos Tsitsipas and Coco Gauff going through to the third round.