Emma Raducanu was not able to see out victory against Dayana Yastremska at the Slovenia Open as her opponent retired when two points from defeat.
Raducanu, returning to action after a first-round loss at the US Open, where she had been the defending champion, looked set to ease past Yastremska.
The Briton did advance to the second round, but only courtesy of a premature conclusion to the match.
Raducanu led 6-2 5-3 and was 30-0 up in her own service game when Yastremska retired, complaining of a wrist injury.
Yastremska has previously been accused of gamesmanship for similar antics; of her now six career mid-match retirements, all have come with her a break down and about to lose, with four coming in a potentially decisive game.
Her decision to quit at this year's Australian Open was particularly unpopular, with Madison Brengle on the brink of inflicting a third-set bagel.
However, Raducanu did not look to criticise Yastremska after this latest contentious finale.
"Obviously no one wants to win by retirement," the top seed said. "But Dayana is an extremely ferocious competitor, so I knew it was going to be a tough match going in."
Fellow seeds Beatriz Haddad Maia, Ekaterina Alexandrova and Anastasia Potapova all also progressed in Portoroz.
At the Chennai Open, only eighth seed Chloe Paquet was subject to a scalp against Karman Kaur Thandi, with Rebecca Peterson and Eugenie Bouchard among the other victors.