Mario Ancic is a doubt for Wimbledon and the Davis Cup clash with Great Britain after pulling out of Queen's.
Ancic has not played since February
The Croat has been suffering from glandular fever and has not played since February.
Ancic would be a key part of Croatia's team to face Britain in the Davis Cup tie at Wimbledon on 21-23 September.
Croatia number one Ivan Ljubicic has confirmed he will play but Ivo Karlovic is also doubtful as he is in dispute with his national federation.
Among the possible replacements would be 6ft 5in Marin Cilic, an 18-year-old who was recently given a wild card for the Artois Championships which begin at Queen's Club on 10 June.