Nats manager Dusty Baker had said Tuesday that Strasburg was too ill to start Wednesday, and Tanner Roark would take the mound as originally scheduled for Tuesday.
But the pitcher showed up at Wrigley Field singing a different tune. Sources told ESPN's Pedro Gomez that Strasburg was on the first bus to the park and told Baker he was feeling much better and could start.
"He didn't feel well yesterday but today maybe a new day," Scott Boras, Strasburg's agent, told ESPN's Jesse Rogers in a text message.
Nationals general manager Mike Rizzo told reporters that Strasburg has experienced flu-like symptoms since his start last Friday. The ace right-hander was on antibiotics and given fluids intravenously.
When asked whether teammates pressured Strasburg to pitch, Rizzo said: "No. He felt obligated when he felt as good as he feels today. He felt much more like himself."
With Strasburg announced, Cubs manager Joe Maddon switched up his lineup card, taking out outfielder Kyle Schwarber and inserting outfielder Jason Heyward. It's a cool and cloudy day in Chicago with the wind blowing in at Wrigley, so runs may be hard to come by. Cubs starter Jake Arrieta benefits from the Gold Glove defense of Heyward.
Game 4 of the NLDS was postponed from Tuesday to Wednesday because of rain, leading to speculation that Game 1 starter Strasburg could take the mound on his regular four days' rest. But Baker cited illness and routine in saying that he was sticking with Roark.
Baker said Tuesday that, in addition to Strasburg, several of his players were ill.
"A lot of my team is under the weather with the change of weather, and the air conditioning in the hotel, and the air conditioning here," he said. "It's just this time of the year for mold around Chicago -- I think it's mold. I mean, I have it too."
Given how dominant Strasburg has been, and given the Cubs hold a 2-1 lead in the best-of-five series, it seemed likely that Baker would opt to skip Roark and go with Strasburg on full rest.
After the All-Star break, Strasburg posted a 0.86 ERA, the second-lowest ever by a pitcher in the second half of a season (minimum 10 starts). Then, in the NLDS opener last Friday, the 29-year-old right-hander was dominant, allowing three hits and no earned runs over seven innings, striking out 10 in a 3-0 defeat.
ESPN's Eddie Matz contributed to this report.