The Nationals (54-36) reopened a six-game division lead on the Mets (47-41) -- matching the season-high deficit the Mets faced after getting swept in a June 27-29 series at Washington.
"I think we've put ourselves in a really advantageous situation," Murphy said. "Six games up in the division, I think that puts us 18 games over .500. We're playing really good baseball right now."
By winning three of four games at Citi Field to close the first half of the season, the Nationals erased any ground the Mets made up earlier on the homestand, which had been highlighted by a four-game sweep of the Chicago Cubs.
The Mets fell to 4-9 against Washington this season.
"We're still in a race," manager Terry Collins said. "The state of the team is we're banged up. That's why we're taking the four days and hopefully recuperate. We've been in this situation one year ago where things looked bleak and we ended up playing in the World Series. So we're upbeat."
Of course, the Mets added Yoenis Cespedes at last year's non-waiver trade deadline. No similar acquisition is expected this time.
"It's a very disappointing end to the homestand. No question about it," Collins acknowledged. "But I'm a big boy. I swallow it pretty good. I can handle it. We've got to get ready for next weekend. When we get into Philly, that's when we've really got to get it going."
Cespedes, who remained unavailable because of a strained right quadriceps, added through an interpreter: "All I can say is there's a lot of baseball left."
Murphy's production against the Mets might end up being historic considering he still has six games remaining in September against his former employer. His homer Sunday gave Murphy seven home runs and 21 RBIs in 13 games against his former club.
That RBI total against the Mets this season already is the most in 56 years by any player against a team for which he played the previous season, according to the Elias Sports Bureau. The Yankees' Roger Maris had 27 RBIs against the Kansas City Athletics in 1960. Maris played for the Athletics during the 1958 and '59 seasons.
The record for homers in a single season against the Mets is 10, shared by Dick Allen (1968) and Willie Stargell (1966). The record for the most RBIs in a single season against the Mets is 28 by Hank Aaron in 1962.
Murphy has the most homers in a single season by a player against the Mets since Chase Utley also had seven in 2009. Murphy has the most RBIs by a player in a single season against the Mets since Ryan Howard also had 21 in 2006.
"I don't think it's just against us," Matz said. "He's hitting like .350. He's a really good hitter. I don't have any answers. Otherwise, I would have not given up a home run to him today."
Murphy's historic homer production during last year's postseason prompted the Mets to make him a one-year, $15.8 million qualifying offer, but they balked at a multiyear commitment and instead brought in Asdrubal Cabrera and Neil Walker to handle the middle infield. The seeds of Murphy's current production with the Nationals nonetheless were planted by Mets hitting coach Kevin Long during the second half of last season.
The Mets obviously underestimated the long-term ramifications of those adjustments.
"Dan Murphy is a student of hitting," Collins said. "He studies it. He loves it. He works at it. What he did in the postseason last year with some of the changes he made, he obviously said, 'This works. And I'm sticking with it.' And he's probably even refined it a little bit more -- the fact that he's on the plate, the fact that he's pulling a little bit more.
"We always knew he had some power. But he was one of those kind of guys that would be happy to take a hit to left field, which he still will in certain situations. But when you get him in certain counts where he's looking to pull, he's now dangerous, because he can hit a homer."
Matz (7-5) was largely solid other than Murphy's two-run shot. The southpaw ultimately gave up three runs, six hits and four walks in seven innings.
The Mets did get a pair of homers from another player they once allowed to depart as a free agent. Jose Reyes had solo homers against Gio Gonzalez in each of his opening two at-bats to pull the Mets to within 2-1 and then 3-2. However, the Mets had only three other hits in the game.
Reyes produced his 17th leadoff homer as a Met, retaking a share of the franchise record alongside Curtis Granderson. It marked Reyes' first two-homer game since Sept. 27, 2011. That was the second-to-last game of Reyes' original tenure with the Mets. He departed that winter for a six-year, $106 million contract with the Miami Marlins.
"It's always a surprise for me when I hit a homer, because I don't hit too many," Reyes said.
Reyes, who just joined the Mets on Tuesday, expressed some disappointment about the timing of the All-Star break.
"Everything for me is starting to come together, and now I'm going to have four days off," Reyes said. "For me I don't want that right now. It's part of baseball."
Said Collins: "I really believe the more Jose is in the lineup, the more he gets accustomed to what's going on at this level, at this speed, after having some time off, I think he's going to be a big factor."
What's next: Jeurys Familia, Bartolo Colon and Noah Syndergaard as well as Collins and his staff depart for the All-Star Game in San Diego, although Syndergaard will not pitch because of arm fatigue. The Mets open the second half on Friday in Philadelphia. Collins hopes Cespedes is ready to return to the lineup that day. He departed Friday's game against the Nationals and did not play the rest of the weekend. "I feel better, but I'm not sure if I'm going to play on Friday," Cespedes said.