In a performance emblematic of a difficult season, Wozniacki, a former No. 1 and last year's Australian Open champion, lost Monday in the first round at Roland Garros 0-6, 6-3, 6-3 to 68th-ranked Veronika Kudermetova of Russia.
"Definitely wasn't the best match I've ever played," said the 13th-seeded Wozniacki, who had only 15 winners to Kudermetova's 40.
The way-up-then-way-down showing by Wozniacki stretched her losing streak to four matches and dropped her 2019 record to 9-8.
Another past Grand Slam champion dropped out of the field Monday when two-time Wimbledon winner Petra Kvitova withdrew because of an injured left forearm.
The left-hander said the arm had been bothering her for some time and during practice Sunday, "suddenly I felt the pain."
"I went to have an MRI, and, yeah, unfortunately, I have a tear in my forearm, which is not great," said Kvitova, who hopes to be ready for the start of Wimbledon on July 1.
In other action, Rafael Nadal began his bid for a record 12th championship in Paris with a 6-2, 6-1, 6-3 victory over 184th-ranked German qualifier Yannick Hanfmann, who played college tennis at Southern California. Nadal was a bit shaky in the very first game, facing four break points, but he saved them all — and didn't face another the rest of the way.
His feared forehand was not at its dangerous best, accounting for more unforced errors (11) than winners (nine).
"I had my match plan and, yeah, some of the things, they didn't work out well," said Hanfmann, who played college tennis at Southern California. "But, I mean, that's why he's as good as he is."
No. 32 seed Frances Tiafoe of the U.S., a quarterfinalist at the Australian Open in January, threw up a couple of times and his game came apart late in a 6-2, 4-6, 6-3, 3-6, 6-0 loss to Filip Krajinovic of Serbia.
"Obviously very depleted and had nothing really in me," said Tiafoe, now 0-4 at Roland Garros.
Other seeded players on the way home included No. 12 Daniil Medvedev, who was eliminated 4-6, 4-6, 6-3, 6-2, 7-5 by Pierre-Hugues Herbert and No. 15 Nikoloz Basilashvili on the men's side, along with No. 18 Julia Goerges on the women's.
Wozniacki's preparation for Paris had been hampered by an injured left calf, which was heavily wrapped Monday, but she said that was not an issue against her 22-year-old opponent. The Danish player also said she was not bothered by her rheumatoid arthritis, an autoimmune condition that can cause pain and swelling in the wrist and other joints.
"I played really well in the first set. I played aggressive. I played the way I wanted to play. And then I think she got very lucky at the start of the second set and took advantage of the opportunities she got," Wozniacki said, her chin resting on her left hand. "And then I think I just lost a little steam in the end."
She made only three unforced errors in that opening set, but 18 over the last two.
Wozniacki's results have not been the sort she is accustomed to. She made it to the quarterfinals and the fourth round in Paris in the last two years.
"You want to win, you're competitive, you work hard and you want to see results. So, it hasn't been a great year for me so far. And I'm just going to try and work hard and try to turn that around. Right now, there's not much I can do about the first six months of the season. I had some good weeks where I felt great, and then I've had some weeks where I've been sick and some weeks where I've been injured," Wozniacki said.
"I just have to try and stay positive, and obviously it's not as easy to stay positive when things aren't going your way," she added. "But I think that's when you really need to — and you need to just keep grinding."