Aliaksandra Sasnovich ended Anna Karolina Schmiedlova's defence of her Bucharest Open title in the first round on Monday.
The Slovak, who defeated Sara Errani in last year's final, was seeded third for the WTA International event on clay at Arenele BNR, but she gave up her serve three times in the opening set and on four occasions in the decider, as Belarusian Sasnovich won 6-4 1-6 7-5.
She will face either Pauline Parmentier or Ysaline Bonaventure for a place in the quarter-finals.
In the other first-round matches in the Romanian capital, Turkish eighth seed Cagla Buyukakcay saw off another Slovak, Kristina Kucova, 6-0 7-5.
Up next for Buyukakcay is American Vania King, who ousted Paula Kania of Poland 7-6 (7-4) 6-0.