On Thursday, organizers announced the field for the April 15-23 event.
The sixth-ranked Federer has yet to announce his full clay-court schedule, having said he would decide after the Miami Open.
The former top-ranked player won a record-tying fifth title in Indian Wells last week to go with his record 18th Grand Slam title at the Australian Open in January. Federer missed most of last year with various injuries.
Monte Carlo tournament director Zeljko Franulovic says he still hopes Federer and the fourth-ranked Nishikori will ultimately decide to join the field.