The Mets improved to 5-2 on their eight-game road trip against playoff contenders, winning for the 16th time in 21 games overall.
Milwaukee's wild-card lead was cut to 4½ games over the Phillies. Still missing injured closer Billy Wagner, New York goes for a three-game sweep Wednesday in this potential postseason preview.
Pinch-hitter Daniel Murphy opened the 10th with a single off Salomon Torres (6-4) and went to third when catcher Jason Kendall made a throwing error on Jose Reyes' sacrifice bunt. Murphy scored easily on Chavez's sacrifice fly to right.
Joe Smith (3-3) got the final out in the ninth and Luis Ayala pitched the 10th for his fifth save in six opportunities since coming to the Mets in a trade with Washington.
Ayala gave up a two-out double to pinch-hitter Brad Nelson, his first major league hit, and walked pinch-hitter Gabe Kapler before striking out Rickie Weeks.
New York lefty Jonathon Niese lasted three-plus innings in his big league debut. The 21-year-old Niese, starting in place of injured John Maine, gave up five earned runs and seven hits to go with four walks and two strikeouts.
Milwaukee leadoff batter Rickie Weeks hit the second pitch from Niese for a home run to left. Niese then walked the next two batters but got out of the inning by stranding runners at second and third.
New York took a 2-1 lead in the second on a run-scoring double by Fernando Tatis and an RBI groundout by Damion Easley.
Beltran, who left Monday's game in the eighth with a bruised knee, gave the Mets a 5-1 lead with his 22nd homer in the third.
New York has homered in 13 straight games.
Milwaukee tied the score at 5 in the fourth after Niese gave up hits to the first five batters of the inning.
Brewers starter Manny Parra hit a two-run double and J.J. Hardy drove in a run with a single that ended Niese's night. Nelson Figueroa struck out Ryan Braun but walked Prince Fielder before Corey Hart tied the game with a single. Hardy, however, was thrown out at the plate on a strong throw from Beltran in center.
Parra tossed five-plus innings, giving up five runs - four earned - and six hits. He struck out six but has just one win in his last eight starts.
