Stan Wawrinka has withdrawn from next week’s ABN Amro World Tennis Tournament in Rotterdam, the tournament announced on Twitter on Tuesday.

The U.S. Open champion is bothered by a knee injury, which also hampered him at the Australian Open last month.

Slecht nieuws: Stan Wawrinka komt door een knieblessure niet naar Rotterdam #abnamrowtt https://t.co/dVAILjn3LB — ABN AMRO WTT (@abnamrowtt) February 7, 2017

Stan Wawrinka withdraws from @abnamrowtt with a knee injury, which bothered him in Melbourne as well. Paire comes in as an alternate. — Franklin Stoker (@StokerSports) February 7, 2017

Wawrinka was active in the season’s first month, winning seven of nine matches. He reached the semifinals in Brisbane, ultimately losing to Kei Nishikori in straight sets, before reaching the semifinals in Melbourne. At the season’s first Slam, the world No. 3 played two five-setters and a four-setter. He succumbed to countryman Roger Federer, 7-5, 6-3, 1-6, 4-6, 6-3, in a classic semifinal.

Wawrinka won this tournament in 2015, defeating Tomas Berdych in the final.

The ABN Amro World Tennis Tournament, played on indoor hard courts, is the first 500-level tournament of the season. Even without Wawrinka, who would have been the top seed, the field in the Netherlands is loaded.

Marin Cilic, Dominic Thiem and Rafael Nadal will headline the draw. Berdych, David Goffin, Jo-Wilfried Tsonga and Grigor Dimitrov are also scheduled to play.