NEW YORK -- When the New York Mets played their home opener against the Philadelphia Phillies on April 8, Robert Gsellman was about 190 miles north of Citi Field in Binghamton getting ready to make his first start of the season the next day for the Mets' Double-A affiliate.

On Sunday, Gsellman capped the Mets' home schedule by becoming the appropriately wide-eyed symbol for an unlikely playoff push that could lead to him making at least one more start at Citi Field.

Gsellman exited to a standing ovation after throwing seven shutout innings Sunday afternoon when the Mets posted the most lopsided shutout win in franchise history by blanking the Philadelphia Phillies 17-0.

"Everybody where this guy's ever pitched, they said his stuff plays, and it does -- we've seen it," Mets manager Terry Collins said after Gsellman improved to 3-2 with a 2.56 ERA in seven games (six starts) since being promoted from Triple-A Las Vegas on Aug. 23. "And now we've got to just certainly hope we get two more starts out of him. At least."

With the win, the Mets (83-73) hold a one-game lead over the San Francisco Giants (82-74) in the race for the first National League wild card. The St. Louis Cardinals (81-74) sit a half-game behind the Giants.

The Mets were in fifth place in the wild-card standings as recently as Aug. 19 but have the best record in baseball (23-11) since then. The run has been fueled by Gsellman and fellow rookie Seth Lugo, who replaced injured left-handers Jonathon Niese and Steven Matz in the rotation. Since Aug. 20, New York is 10-3 in games in which either Gsellman or Lugo have appeared.

Gsellman seemed to be absorbing a whirlwind six weeks as he walked off the mound in the seventh inning with a small grin and a gaze that traveled around the upper reaches of Citi Field.

"Pretty crazy -- it's awesome," Gsellman said.

And it could get crazier. The Mets' two non-rookies in the rotation, Bartolo Colon and Noah Syndergaard, are lined up to start the final two games of the regular season against the Phillies next Saturday and Sunday. Should Colon and Syndergaard be unavailable if New York advances to the National League wild-card game on Oct. 5, the start would fall to either Lugo or Gsellman, who would be on his regular rest.

"Kind of came pretty fast," Gsellman said of a possible playoff start. "Definitely having fun so far."

Asdrubal Cabrera hit a grand slam in the seventh inning. Jose Reyes (double), Eric Campbell (single) and Michael Conforto (double) each had two-run hits during a six-run eighth for the Mets, who trailed 10-0 after four innings Saturday but outscored the Phillies 25-0 the remainder of the series.

Curtis Granderson homered in the fourth and T.J. Rivera had a single in the fifth for the Mets, who scored five runs via two bases-loaded walks, a bases-loaded hit batsman, a bases-loaded wild pitch and a run-scoring double play.

The margin of victory was the second biggest in Mets history, behind a 19-1 win over the Chicago Cubs on May 26, 1964. The previous biggest shutout in franchise history was a 14-0 win over the Cincinnati Reds on April 19, 1998.

Jake Thompson (3-6) took the loss after allowing three runs, five hits and three walks while striking out one in four innings for the Phillies (70-86), who suffered the franchise's most lopsided loss since a 22-3 defeat at the hands of the the Reds on Sept. 4, 1999. The shutout was the most lopsided since at least 1913, the earliest year for which Baseball-Reference.com has data.

"Pretty embarrassing game today," Phillies manager Pete Mackinaw said. "Tough series."

The Mets' win capped a somber day in which both teams played while mourning the death of Miami Marlins pitcher Jose Fernandez, who was killed in a boating accident early Sunday morning.

"You've still got to go out, you've still got to play," Phillies first baseman Ryan Howard said before the game. "But I think that everybody in baseball today has a heavy heart."

The Mets, who were scheduled to face Fernandez in the opener of a three-game series in Miami on Monday, hung a Mets jersey with Fernandez's name and number 16 on it. The jersey was placed in the dugout by outfielder Yoenis Cespedes, who, like Fernandez, fled Cuba for the United States.

Cespedes said the Mets would also put the jersey in their dugout on Monday, when the Mets-Marlins game will go on as scheduled.

"I wanted to be the one to go out there and put it up, just to let everyone know how much this loss meant to us," Cespedes said through an interpreter.

NOTES: Jose Fernandez made his major league debut against the Mets at Citi Field on April. 7, 2013. "When the first pitch left his hand, the first thought is 'oh wow, this is something special,'" Mets manager Terry Collins said. ... Phillies 1B Ryan Howard had more plate appearances (14) against Fernandez than any other current Phillies player. "I loved his spirit," Howard said. "I loved watching him pitch, competing against him, because he was that guy. He was a competitor. He wanted to win."