Juan Martin del Potro is practicing without a knee brace ahead of his Argentina Open comeback.
"I want to come back strong and give it everything that I have," Roger Federer said.
The Austrian suffered an injury to his right hand and needs time to recover.
Former Delray Beach resident Tommy Paul returns to his stomping grounds this month.
The two legends will be reunited in London in September
U.S. captain Mardy Fish also names Jack Sock and Rajeev Ram to squad.
Daniil Medvedev's chase for the world No. 1 ranking will have to wait.
