Monte Carlo - Spain's seventh seed David Ferrer withdrew on Tuesday from the Monte Carlo Masters a day prior to his second-round match against German teenager Alexander Zverev.
ATP officials only announced a problem with the 34-year-old Spaniard as "an injury" with no further details.
Ferrer, a finalist in the Principality five years ago against Rafael Nadal, has had a patchy 2016 season, taking a break from late February to late March to rest.
Upon his return, the ATP number eight lost in the Miami third round to France's Lucas Pouille.