Messi, who has been nursing a sore back after sustaining an injury in a friendly against Honduras last month, has trained separately under the supervision of team medical staff since arriving in the United States.
However, the 28-year-old Barcelona superstar took part in a full training session on Wednesday at the University of Illinois campus, two days after sitting out Argentina's opening 2-1 victory over Chile in Santa Clara on Monday.
In a short message on his Facebook page, Messi said he hoped to make a swift return to Argentina's starting line-up.
"I hope to be ready as soon as possible to help the team," Messi wrote.
A victory against Panama on Friday would almost certainly guarantee a place in the quarter-finals for Argentina with one game remaining in Group D.
Argentina are desperate to win this year's Copa America as they aim to end a 23-year-old title drought. That barren spell has included defeats in the 2014 World Cup final and last year's Copa America final.