Keys, an 18-year-old who was born in Rock Island, Ill., saved the only break point she faced while converting 3 of 10 on Barthel's serve.
The 52nd-ranked Keys also defeated Barthel on grass in a tuneup tournament at Birmingham, England, two weeks ago.
It's the second time in the past three Grand Slam tournaments that Keys has made it to the third round. She lost at that stage at the Australian Open in January.
Bidding to reach the round of 16 for the first time at a major championship, Keys will face either 2012 Wimbledon runner-up Agnieszka Radwanska, who is seeded fourth, or Mathilde Johansson.
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