Tennis players love Paris. They’re less sure about Parisians.

What Swiatek had discovered was the same thing that generations of visitors have encountered on trips to the French capital: Parisians can be kind of rude.

“Please guys,” Swiatek implored the supporters inside Philippe-Chatrier Stadium, “if you can support us between the rallies, but not during, that would be amazing.”

Swiatek isn’t the only one who’s noticed fans here behaving with more contempt than a café waiter.

Less than 24 hours earlier, Belgium’s David Goffin had plenty to say about how he’d been treated on Court 14. Granted, he was facing France’s Giovanni Mpetshi Perricard in front of a hometown crowd, but Goffin insisted that the booing, jeering, and constant heckling from some fans in the front row was over the line. “I think this only happens in France,” he added.

At one point, Goffin added, someone even spat gum at him.

“I played 20 years ago and there could be some hostile crowds, but not like today,” seven-time major champion Justine Henin said on French television.

It’s no secret in the tennis world that Grand Slam tournaments are rowdier than they used to be. Nights at the U.S. Open and Australian Open now offer up a heady cocktail of long sessions and ample access to bars. The result usually ends up ringing in players’ ears, which they love when it comes at appropriate moments. The rest of the time—such as on a ball toss or a close line call—it leads to stony glares toward the crowd and exasperated pleas from the chair umpire.

“Please don’t call the ball out during rallies,” the umpire had to say at one point during Swiatek’s match. “We have line judges for that.”

Swiatek’s opponent Naomi Osaka, however, said she had no problem with the noise on Wednesday. Then she added, “But I’m also used to the New York crowd.”

It takes on an extra dimension here whenever a French person is on court. That’s because home fans haven’t had a men’s champion since Yannick Noah in 1983 or a women’s champion since Mary Pierce in 2000. So their opponents, no matter where they’re from, always end up with an earful.

American Taylor Fritz felt it firsthand last year after eliminating France’s Arthur Rinderknech under a shower of boos. He only egged on the fans by shushing them.

Swiatek wasn’t as confrontational. The three-time champion made a point of keeping the locals onside.

“I hope you’re still going to like me, because I know that French crowds get some players that they don’t like and then boo,” she said. “But I love you guys and I always love playing here.”

Write to Joshua Robinson at Joshua.Robinson@wsj.com