"He feels strong and, like I said, he's throwing the ball where he wants to throw it," Martinez said. "So it's good. I'm looking forward to watching him go out there and compete come April 7."
Corbin, 32, enjoyed the best year of his career in 2019, his first with Washington. He went 14-7 with a 3.25 ERA while helping pitch the Nationals to their lone World Series title.
The left-hander struggled each of the past two seasons, going 11-23 combined. Last season, his ERA ballooned to 5.82.
Martinez says he's seeing a different pitcher this spring.
"The biggest thing I noticed from him is he's not fighting himself as far as mechanics-wise," Martinez said.
Corbin's final scheduled spring start Saturday moved up a few hours and to the backfields of the Ballpark of the Palm Beaches in an effort to avoid rain forecast for the evening. He tossed 90 pitches in six innings.
"He threw the ball well," Martinez said. "He felt good. Like I said, he had a good camp and some of the things that we talked to him about at the end of the year last year, he really came in this year and worked on a lot of things this winter and he came back."
Corbin signed a $140 million, six-year deal to join the Nats in 2019.
"I want him to go out and compete and just go have fun and be the guy that he was in '19, and before then," Martinez said.
Ace right-hander Stephen Strasburg won't be ready to start the season as he works back from surgery for thoracic outlet syndrome. The Nationals slowed Strasburg's workload earlier this spring, but he is now back throwing bullpens. He hasn't pitched in a Grapefruit League game.
Another projected starter, Erick Fedde, is bothered by an oblique injury but could still start the season in the Nationals' rotation.
Washington re-assigned Cade Cavalli to minor league camp on Saturday, meaning 36-year-old right-hander Anibal Sanchez will likely begin the season in the rotation, as will Josh Rogers and Josiah Gray. Washington brought Sanchez to camp on a minor league deal.
Joan Adon, a 23-year-old right-hander, was slated to fill in as the Nationals' Saturday night starter against Miami, ultimately canceled by rain, and could find himself in the rotation if any of the above pitchers suffers a setback.