The 2015 winner Stan Wawrinka has withdrawn from next week's Rotterdam Open with a right knee injury, leaving Rafael Nadal as the main drawcard for the indoor hardcourt ATP event.
The world No.3 carried his knee problem throughout the Australian Open and wore strapping until his quarter-final loss to ultimate champion Roger Federer.
Wawrinka won the Dutch title two years ago over Tomas Berdych.
"Stan called me this afternoon to share the bad news," tournament director Richard Krajicek said.
"Test results today showed that he is not able to play matches as a result of his knee. The medical advice came as a complete and unpleasant surprise - not just for us, but for him as well."
Wawrinka earlier called the knee worry "sure an issue since the beginning of the tournament (Australian Open)".
"Then again, it's not an excuse at all. I always try to fight on the court, to find a solution," he said.
"I go on the court with what I have of the day. I give everything. What exactly I have, I have to check. Really I have to do some bigger test when I get home."
All of the spotlight falls on Australian Open finalist Nadal, who lost to Federer in the decider.
Others entered in the Dutch port city include Berdych, Croatian Marin Cilic, Jo-Wilfried Tsonga of France and Bulgarian Grigor Dimitrov.
