Mejia pitched seven scoreless innings in his season debut and Daniel Murphy homered twice and tied a career high with five RBIs and the Mets defeated the Washington Nationals 11-0 Friday in the opener of a day-night doubleheader.
Murphy went 4 for 5 and added a pair of RBI singles. Juan Lagares had three hits and Ike Davis added a three-run homer for New York, which has won five of seven.
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Mejia (1-0), who had been on the disabled list since March with shoulder inflammation, allowed seven hits. He struck out seven with no walks.
"I haven't seen him obviously that good in a long time," manager Terry Collins said. "The command of his stuff was very, very good. His location. I thought he pitched an outstanding game. His best changeup. I didn't know he had one. He used it and used it effectively. He pitched a great game."
Mejia, 23, was 0-4 for the Mets in 2010. He underwent Tommy John surgery in 2011 and spent most of 2012 at Triple-A Buffalo. He was 1-2 with a 6.33 ERA in three September starts for the Mets.
Prior to being recalled for Friday's start, the right-hander went 2-0 in a pair of rehab starts at Double-A Binghamton.
"Certainly it's easy to fall back in the pack," Collins said of Mejia, whom he said will remain with the Mets when they make a roster move after the doubleheader.
"He was the rage in 2010. Everybody thought this guy's, the sky's the limit. And then the injuries have set him way back.
"I give our rehab coordinator, Jon Debus, tremendous credit. When he was rehabbing, (Debus) said, `There's no reason why your name shouldn't be in the hat. At one time, you were the guy. You were the big dog. Now they're talking (Zack) Wheeler, (Matt) Harvey.' And he said, `There's no reason why your name shouldn't be in the mix.' And I think today he showed you it should be in the mix."
Steve Lombardozzi and Ryan Zimmerman had two hits apiece for Washington, which was held scoreless a day after its dramatic 9-7 walk-off win over Pittsburgh.
"That means a lot," Mejia said of his performance. "When I was here the first day that I pitched in September last year, I threw like a reliever. But now I come here and my first pitch is like a starter. And I feel I can be a starter now."
Jordan Zimmermann (12-6), who allowed seven runs in a career-low two innings Sunday in a 9-2 loss to the Dodgers, needed 118 pitches to go 6 2-3 innings.
He allowed five earned runs and six hits while striking out eight and walking three in picking up his third straight loss.
"I thought he threw the ball good," catcher Kurt Suzuki said. "It was one guy that kind of ... Murphy kind of killed us."
With one out in the first, Murphy hit a 1-1 slider off the second deck façade in right center to make it 1-0.
In the third, after Lagares doubled with two outs, Murphy put New York up 3-0 when he lined a pitch into the Nationals bullpen for his eighth homer.
"I felt like, you know, I pitched a lot better than what the stat line says, I made two mistakes and three runs," Zimmermann said. "And I've thrown those pitches to Murphy plenty of other times and gotten him to ground out to second. Today just wasn't my day."
New York added two runs in the seventh. Lagares singled home Anthony Recker, who stole second and scored on a single by Murphy.
The Mets made it a rout with a six-run ninth capped by Davis' homer.
Murphy has hit safely in 15 of his past 16 games.
"You look and see what David (Wright) has done all year and we've kind of been riding Marlon's (Byrd) back, it's nice to feel like you're really able to contribute as well," Murphy said.
The Nationals had base runners in each of the first four innings against Mejia, but went 0 for 4 with runners in scoring position.
After Washington's Bryce Harper singled in the eighth, he was removed for pinch runner Scott Hairston and was not in the lineup for the second game. Johnson said Harper aggravated his knee diving for a ball.
NOTES: LF Eric Young Jr. was scratched from the Mets lineup after a pregame workout to test his right knee. He injured the knee Wednesday on the play in which Tim Hudson fractured his right ankle and left Thursday's game against Atlanta in the fourth inning. ... It was Murphy's second career multi-homer game. His first was June 27, 2012, against the Cubs. ... Zimmermann's eight strikeouts gave him 500 for his career. ... Washington activated RHP Ryan Mattheus from the disabled list. The reliever missed 58 games with a fractured right hand.
33,858 (81.6% full) - % is based on regular season capacity
78 degrees, cloudy
Home Plate - Wally Bell, First Base - David Rackley, Second Base - Lance Barksdale, Third Base - Vic Carapazza
Mets outfielder Daniel Murphy's first-inning home run was calculated at 444 feet, the longest of his career. It is the longest home run hit at Nationals Park since Sept. 8, 2012, when Bryce Harper hit one 444 feet off Mark Buehrle.