From Comcast SportsNetNEW YORK (AP) -- The season is off to an unexpected start for the New York Mets. They're the ones getting big hits while their opponents make the crucial mistakes.Daniel Murphy singled home the winning run in the ninth inning and the undefeated Mets took advantage of a throwing error by reliever Henry Rodriguez to beat the Washington Nationals 4-3 on Monday night.Kirk Nieuwenhuis hit his first major league homer for the Mets, who are 4-0 for the first time since 2007. After a surprising sweep of Atlanta, New York rallied from a three-run deficit before a crowd of 23,970. Several fans filed out chanting "Undefeated! Undefeated!""Everybody is excited. We know that it's a long year, but we want to show our fans that maybe we are better than everyone expects us to be," manager Terry Collins said. "It's never about the effort with these guys."Coming off three straight losing seasons since Citi Field opened, the Mets were projected by most to finish last in the NL East this year. But they did upgrade the bullpen last winter and they received another excellent effort Monday from a retooled unit that ranked 28th in the majors in 2011 with a 4.33 ERA.Miguel Batista got out of trouble in the sixth, Ramon Ramirez escaped a seventh-inning jam with a double-play ball and Jon Rauch (1-0) worked two hitless innings for his first win with New York. Mets relievers are 2-0 with a 0.68 ERA in 13 1-3 innings.Pinch-hitter Mike Baxter drew a leadoff walk from Rodriguez (0-1) in the ninth and Ruben Tejada sacrificed with two strikes. Rodriguez looked at second, then threw low to first and the ball got by second baseman Danny Espinosa."I'm strong, so sometimes I throw sidearm and the ball moves," Rodriguez said.New third base coach Tim Teufel initially waved Baxter all the way around, but he threw up a late stop sign and Baxter slipped to the turf as he tried to slam on the brakes halfway down the line. He got back to his feet and scrambled back to third, barely beating Espinosa's perfect throw across the diamond."The only thing I wanted to make sure is, I didn't want to make that long throw, you know, throw behind him and have him get up and go straight (home)," Espinosa said. "So I wanted to make sure he had kind of a step to commit over there."Espinosa was shaken up after catching an elbow in the head from Tejada as he ran through the bag, but the Washington second baseman stayed in the game."It's just one of those crazy plays where if you just execute from the beginning, it's a little bit better. But things happen," Nationals third baseman Ryan Zimmerman said.With first base open and David Wright on deck, the Nationals went after Murphy, who made a diving play at second base to end the top of the ninth. He fisted a looping single to right over a drawn-in infield that dropped in front of Jayson Werth.New York was 1 for 10 with runners in scoring position before Murphy came through. The Mets mobbed him near first base and a teammate pelted him in the face with a cream pie as he was interviewed on the field.Adam LaRoche extended his fast start with a pair of RBI singles for the Nationals. Washington fell to 2-2 in its first full season under Davey Johnson, who managed the Mets to their most recent World Series championship in 1986.Edwin Jackson squandered an early three-run lead in his Nationals debut and was pulled for a pinch-hitter after five innings. The right-hander signed an 11 million, one-year contract after helping St. Louis win the World Series last season.Looking for a bounce-back season, Mets starter Mike Pelfrey gave up 10 hits over 5 2-3 innings in his first outing of the year. But he struck out eight, matching a career high."If I can take that stuff out there every single time, it's going to be a good year," Pelfrey said.Nieuwenhuis was called up after newly acquired center fielder Andres Torres re-injured his calf on opening day. The 24-year-old outfielder had two hits in his big league debut Saturday and hit a two-run shot in the fourth inning Monday to tie it at 3."It could be the start of what might be a very good major league career," Collins said. "After he tied it up, the intensity in the dugout picks up."The drive to right, estimated at 385 feet, cleared the new fence at Citi Field and clanked off the old one, making it the second home run in four games (both hit by the Mets) that would have stayed in the ballpark under the previous configuration.Nieuwenhuis' parents were at the game, and he got the home run ball back as a souvenir. He plans to give it to his father -- even though it was his mother's birthday.Trailing 3-0 and booed as he stepped to the plate, the 6-foot-7 Pelfrey sparked New York's offense in the third. He ripped his fifth career double into the left-field corner and scrambled to third on Tejada's long flyout. Wright's two-out single made it 3-1.NOTES:Nationals LHP Tom Gorzelanny tossed 2 2-3 innings of scoreless relief in his season debut. ... New York 1B Ike Davis is 0 for 15.