Rendon played five innings in Florida Wednesday, while Doug Fister and Sammy Solis began throwing in Potomac.
The 26-year-old lefty has pitched well since his late-April call up, and earned his first big league victory in the Nationals 5-4 win Sunday.
The rookie left-hander and three other Nationals relievers combined for an impressive showing that kept Washington in Tuesday night's game until the end.
Elbow, back and shoulder injuries slowed the left-hander's ascent to the majors, but Solis looked good in relief on Thursday night.
With the bullpen needing reinforcements and Reed Johnson injured Tuesday, the Nationals made moves to stabilize their roster.
The 25-year-old's professional baseball career has been slowed by injuries, the latest an elbow issue as he returned from a back one.
Both Ian Desmond and Bryce Harper are dealing with a cold.
Sammy Solis can finally focus on pitching again, plus other notes from the Nationals' workout.
"It’s up to the team," he said. "I’m a starting pitcher, but I’ll relieve, catch, whatever."
The Nationals may turn to Sammy Solis in the bullpen, a way to improve their relief and give a top starting prospect experience.