World number 11 Mario Ancic faces two months on the sidelines after being diagnosed with glandular fever.
Ancic has struggled for form all season
The Croatian has a mild case of the illness but seems certain to miss the Masters Series events in Miami and Indian Wells.
But he is expected to recover in time to play at the French Open, which gets under way at Roland Garros on 27 May.
Ancic has only won four matches this year, the last coming at the Australian Open where he reached the last 16.