World number 2 Osaka cruised through the first set and looked like she would make short work of the Swiss wild card but soon Teichmann swung the momentum firmly in her favour.
An error-prone Naomi Osaka has been stunned 3-6 6-3 6-3 by Jil Teichmann in the third round of the Western & Southern Open in Cincinnati.
World number two Osaka cruised through the first set and looked like she would make short work of the Swiss wild card.
But world number 76 Teichmann settled down and took the second set, and the momentum swung firmly in her direction in the fourth game of the decider when Osaka missed overhead smashes on back-to-back points, leading her to scream in frustration.
The miscues continued to pile up for the four-times Grand Slam champion, who issued her 41st unforced error on match point when she sent a service return long as Teichmann advanced to the quarter-finals with the biggest win of her career.
"I'm shaking right now," Teichmann said.
"She's a very tough opponent, a Grand Slam champion, and I'm the underdog, which is a bit easier. But you've got to start somewhere."
Osaka will look to find her form ahead of the upcoming US Open in New York, where she is a two-time champion.