Various ATP players sidelined with injury continue to press toward a comeback.
Del Potro, who underwent surgery last month, is expected to be out for about six months and could miss the rest of the season.
Janko Tipsarevic, who spent most of last year struggling with a heel injury, is planning to play at Dusseldorf the week before the French Open.
"I'm very happy to be back on court," he said in a statement from the tournament. "Dusseldorf is a simply great tournament. I am excited."
Jurgen Melzer, who has spent six months sidelined with a shoulder injury, plans to return at Monte Carlo next week.
"The shoulder is obviously not like new, but is good enough to play tournaments," said the Austrian, who wants to return to the Top 50.