The Bulgarian third seed, playing only his third Tour match since a first-round exit at the US Open, struggled to contain a tricky opponent who deservedly pocketed his first victory over a top-10 player for four years.

Eighth seed Kyle Edmund maintained his strong run of form as he battled past Argentina’s Diego Schwartzman in his opening match, three days after beating Gaël Monfils to register his first ATP Tour title in Antwerp.

The British player won 6-3 7-6(3) to set up a last-16 clash with Spanish veteran Fernando Verdasco who scraped past Frenchman Pierre-Hugues Herbert after losing a first-set tiebreak the previous day.

Italian Fabio Fognini was bundled out in the second round in three sets by Hungary’s Márton Fucsovics who now faces either Kukushkin or young Russian Andrey Rublev.

Another in-form Russian, Karen Khachanov, beat Austrian wildcard Dennis Novak 6-3 7-5.