Aug 19 (Reuters) - An error-prone Naomi Osaka was stunned 3-6 6-3 6-3 by Jil Teichmann in the third round of the Western & Southern Open in Cincinnati on Thursday. 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 U.S. Open in New York, where she is a two-time champion. Reporting by Rory Carroll in Los Angeles; Editing by Peter Rutherford Our Standards: The Thomson Reuters Trust Principles.