Slovak wild card Mia Pohankova, the 17-year-old who won Wimbledon Junior last year, came from 4-1 down in the second set to upset No. 2 seed Clara Tauson in the first round of the Libéma Open.
It was the sixth successive defeat for Tauson, who hasn't won a match in three months.
The Dane, who was the 2nd seed in the tournament, was otherwise up 4-1 in the second set but was broken twice as she completely lost her rhythm, ultimately losing 6-4.
There are still three weeks until the next Grand Slam of the year, Wimbledon.