Belgian world number 10 David Goffin will miss the ATP event in Geneva, which starts on Saturday, organisers announced Wednesday.
The 27-year-old would have been the top seed in Switzerland, which is the final tournament before the start of the French Open on May 27, and is currently playing at the Italian Open.
Goffin reached the 2016 French Open quarter-finals and was runner-up to Grigor Dimitrov at last year's ATP Tour Finals.
His withdrawal leaves American world number 12 Sam Querrey as the highest-ranked player in the draw for Geneva, but tournament organisers said Goffin would be replaced by another high-level player.
Three-time Grand Slam champion Stan Wawrinka could ask for a wildcard as he continues his comeback from injury, but according to his agent he is yet to decide between Geneva and the concurrent tournament in Lyon.
Goffin, who was beaten by Britain's Kyle Edmund in the Madrid Open third round last week, faces fifth seed Juan Martin del Potro in the Rome last 16 on Thursday.
He has been hit with two freak injuries in the last 12 months -- twisting his ankle when tripping on tarpaulin at Roland Garros last year and suffering an eye injury after being struck by a ball in Rotterdam in February.