Federer was forced to skip the French Open because of a lingering back injury.
Stuttgart tournament director Edwin Weindorfer says Federer is scheduled to practice on Sunday.
The former No. 1 had signed up for the inaugural grass-court edition of the tournament that used to be played on clay. Now, it is a warm-up for Wimbledon.
Federer is a seven-time Wimbledon champion. After Stuttgart, he's scheduled to play in Halle, another grass-court event he's won a record eight times.