Kei Nishikori was not listed in the ATP rankings for men's singles on Monday for the first time in 12 years after injuries ruled him out of tour appearances over the past 12 months.
The former world No. 4 had arthroscopic left hip surgery in January and has not played in tour events since last October. The last time he was not in the ranking was in April 2010.
Earlier this month, Nishikori said it remains unclear whether he can return to the ATP Tour by the end of the year.
The 32-year-old was ranked 759th in the world last week.