"Harvey's better!" they chanted, over and over.
"It was nice," Harvey said, "but we've got a long way to go."
Harvey and Strasburg paired off for the first time in their careers, with many projecting the celebrated 24-year-olds will duel far into the future. Harvey (4-0) was equal to the challenge and started out fast, striking out leadoff man Denard Span with 98 mph heat.
"I came out a little too pumped up," he said.
Davis and Duda hit solo home runs in the sixth -- the first time Strasburg (1-3) ever had been tagged twice in an inning -- for a 4-0 lead. The 26,675 fans at Citi Field broke into the "Harvey's better!" chants that bounced around the ballpark.
"There's going to be a lot of years you're going to talk about this guy," Mets manager Terry Collins said.
Davis added a two-run homer in the eighth and Duda hit a solo drive off Drew Storen.
Harvey gave up one run and four hits in seven innings, striking out seven. He's been so dominant, his ERA actually went up, from 0.82 to 0.93.
Strasburg allowed four runs, two unearned, and five hits in six innings. He fanned six, flinging fastballs in the upper 90s.
Strasburg played down any extra emphasis on opposing Harvey.
"It's funny, because you guys want to make a big deal out of all this, but every game's huge for me. I want to go out there and want to help this team and it doesn't matter who's facing us," he said.
As for Harvey, "he's got good stuff. He's pounding the strike zone. It's still early, obviously, a lot of our guys haven't really faced him that much," Strasburg said.
Harvey gave up just one hit through five innings, a sliced double by Strasburg, before running into his only jam in the seventh. A leadoff walk and singles by Ian Desmond and Chad Tracy made it 4-1, and a throwing error by second baseman Daniel Murphy loaded the bases with no outs.
Collins wasn't about to pull Harvey, even though the pitcher's spot was due up first in the bottom half of the inning.
"I knew I needed to make a pitch," he said.
Harvey showed composure far beyond his years, quickly getting ahead of Kurt Suzuki and then making him chase a slider for strike three. Pinch hitter Roger Bernadina was next, and Harvey got him on a foul popout.
With the crowd chanting his name, Harvey ended the inning by retiring Span on a routine grounder. Harvey watched the play, pumped his fist and got a fist bump from catcher John Buck outside the dugout.
"He pitched great, but that's the mark of a true ace," Collins said. "Games like that can spread to great seasons."
Moments later, the crowd cheered again when the video board posted a message that the second Boston Marathon bombing suspect was in custody. That news prompted some fans to chant "U-S-A!"
Harvey and Strasburg each stand 6-foot-4, and their stature as right-handed flamethrowers loomed large as many around the majors touted this matchup.
With fog swirling around the top of the scoreboard at the start, the Mets took a 2-0 lead in the first. An error by shortstop Desmond set up Strasburg's wild pitch that scored a run, and Buck added a single for his 20th RBI this season.
Harvey grew up in Connecticut and recently said he'd heard about those crackling Friday nights in the 1980s when Mets phenom Dwight Gooden pitched at Shea Stadium. Doc was at Citi Field for this game and gave Nationals manager Davey Johnson -- the Mets' skipper back then -- a big hug behind the cage during batting practice.
"Good to be here," Gooden told a fan on his way toward the field. "Fun night."
Said Harvey: "When I was younger, I wanted to be that guy."
"For him to come out for one of my starts, it was kind of mind-boggling," he said.
Nationals 3B Ryan Zimmerman didn't start because of tightness in his left hamstring. SS Danny Espinosa didn't start because of a bruised right forearm that has sidelined him since Sunday. Span returned to the lineup after missing two games because of an ill stomach. ... The Mets stopped an eight-game home losing streak to the Nats.