NEW YORK -- "Six Degrees of Matt Harvey" might not have the same ring to it as "Six Degrees of Kevin Bacon." But it might just be enough for the New York Mets to return to the playoffs.

Robert Gsellman, the latest pitcher to occupy Harvey's rotation spot, earned his first win as a starter by throwing six solid innings Saturday night as the Mets moved closer in the National League wild-card race with a 3-1 win over the Washington Nationals at Citi Field.

Gsellman is the fourth pitcher the Mets have tried out since Harvey's season ended in early July with a diagnosis of thoracic outlet syndrome. First up was Logan Verrett, who produced a 7.18 ERA in seven winless starts before he was replaced by left-hander Jonathon Niese.

But Niese lasted one start and one out of a second on Aug. 23 before suffering a knee injury and giving way to Gsellman, who earned the win with 3 2/3 shutout innings to earn a promotion to the rotation beginning last Sunday.

"We don't want to keep belabor(ing) the injury issue, but we've asked a lot of some young players and they have stepped up," Mets manager Terry Collins said. "This is what we have."

The Mets are scheduled to send another rookie pitcher to the mound Sunday night, when right-hander Seth Lugo -- pitching in place of injured left-hander Steven Matz -- makes his fourth career start.

Gsellman and Lugo are 4-2 with a 2.70 ERA in six games (five starts) since Aug. 19, a stretch in which the defending NL champion Mets (70-66) have gone 10-5 to move within one game of the St. Louis Cardinals in the race for the second wild card.

"These guys, I think they're caught up in it a little bit," Collins said. "They were in the organization last year when things were going great up here. We've asked them to step up and this is their opportunity."

Gsellman wriggled out of trouble all night in allowing the one run on six hits and three walks while striking out four. He didn't have a single 1-2-3 inning but stranded eight baserunners while also picking off Bryce Harper to end the fifth.

The Nationals loaded the bases with one out in the first, but Gsellman limited the damage by getting Anthony Rendon to hit a sacrifice fly and retiring Wilson Ramos on a pop-up.

"Starting a game like that with bases loaded and one out, the game can change just like that in the first inning," Gsellman said. "I thought I did well to get out of it with one run."

The Nationals had two on with none out in the sixth, but Gsellman ended his night by getting Ryan Zimmerman to ground out, striking out Danny Espinosa and inducing pinch hitter Ben Revere to ground to second.

"It was kind of a nothing game when we didn't come through all game," Nationals manager Dusty Baker said.

Four Mets' pitchers combined to get the final nine outs, with Jeurys Familia notching his 45th save by throwing a perfect ninth. But the biggest relief outs were recorded by another rookie, Josh Smoker.

The left-hander entered with one on and one out in the seventh but got Daniel Murphy to line out to center, where Michael Conforto made an impressive diving catch, before striking out Harper, the defending MVP.

"I know we've had a lot of injuries and guys have stepped up and came through," Smoker said. "It's cool to be a part of."

Curtis Granderson hit a two-run single in the third while James Loney laced an RBI double in the sixth for the Mets. The hit by Granderson was his first in his last 23 at-bats with runners in scoring position dating to July 24 and the first time he collected an RBI with anything other than a homer or a sacrifice fly since July 6. Loney's double was his first extra-base hit and sixth RBI since Aug. 1.

Jayson Werth and Ryan Zimmerman had two hits apiece for the Nationals (79-56), who had a four-game winning streak snapped. Right-hander Tanner Roark (14-8) took the loss after allowing two runs on four hits and four walks while striking out three over five innings.

"When he left, it was 2-1, and we had action on taking the lead," Baker said. "We had quite a few opportunities."

NOTES: The ceremonial first pitch was thrown by gymnast Laurie Hernandez, who won a gold medal in last month's Olympics in the team all-around. Hernandez, holding the baseball, did a forward flip off the mound before firing a one-hopper to Mets C Rene Rivera. ... RHP Rafael Montero, who threw 1 1/3 innings for Double-A Binghamton on Saturday, will likely start for the Mets in place of RHP Jacob deGrom (right forearm) against the Cincinnati Reds on Tuesday. ... The Nationals scratched RHP Max Scherzer from his scheduled start Sunday in order to give him an extra day of rest. RHP Reynaldo Lopez will make the start. ... Nationals 2B Daniel Murphy, who spent the first eight seasons of his career with the Mets, singled in the first inning and has a hit in his first 15 games against the Mets, tying a record set by Hall of Famer Tony Gwynn.