DENVER -- The Pittsburgh Pirates crept closer to clinching a spot in the National League playoffs Monday night, beating the Colorado Rockies 9-3 at Coors Field.

The Pirates' magic number for making the playoffs is two. They could assure themselves of playing in the postseason by beating the Rockies on Tuesday night and having the San Francisco Giants and Washington Nationals both lose.

Pirates left fielder Starling Marte tied his career high with four hits and drove in two runs with a first-inning homer and sixth-inning double. Third baseman Aramis Ramirez also homered in the first off Rockies starter Jon Gray (0-2), who had allowed two homers in 36 innings in eight previous starts.

Shortstop Jordy Mercer matched his career high with four hits, including two run-scoring doubles, as the Pirates won their third consecutive game following a four-game skid.

A.J. Burnett (9-5) held the Rockies to three runs, two earned, and threw 98 pitches in 5 2/3 innings before five Pittsburgh relievers combined to hold the Rockies scoreless for the final 3 1/3 innings. It was Burnett's third start after missing six weeks due to right elbow inflammation.

"Every outing I think he's gotten better," Pirates manager Clint Hurdle said. "We spiked the pitch count up today close to 100. We'll see how he feels tomorrow, but I think there's a lot of good signs he's regaining form. I think it gives us a real good shot in the arm for our rotation."

Burnett allowed six hits and four walks while striking out five. He left with the bases loaded and two outs in the sixth, but Antonio Bastardo got center fielder Charlie Blackmon to hit an inning-ending flyout.

The Rockies, trailing 6-3 at the time, left the bases loaded for the second inning in a row. Burnett struck out Arenado to end the fifth, an inning that should have been much easier for the 38-year-old pitcher.

The Pirates should have turned a double play that would have left the Rockies with two outs and the bases empty, but first baseman Pedro Alvarez dropped an easy throw. Left fielder Corey Dickerson singled off the right field wall, and catcher Tom Murphy followed with his second career homer in his fifth game.

"I thought he dealt all through up into the fifth," Hurdle said of Burnett. "We weren't able to put the double-play ball away. It tacked on 18 more pitches. I think the fatigue of the inning took something away from him. He threw 30 pitches in that inning. That's tough duty here at altitude."

Burnett's five strikeouts gave him 2,499 for his career, leaving him on the verge of becoming the 32nd pitcher in major league history with 2,500 strikeouts. Hall of Famer Christy Mathewson ranks 31st with 2,507 strikeouts.

Burnett trails only CC Sabathia (2,568) among active pitchers in strikeouts.

Mercer, who went 4-for-5, entered the game 16-for-72 (.223) in 23 games since coming off the disabled list Aug. 23 following a left knee injury that caused him to miss 30 games. He doubled home catcher Francisco Cervelli in the second and again in the fourth, putting the Pirates ahead 5-0 at that point, and singled in the sixth and ninth.

Mercer was never on the disabled list before getting injured in July. He said he was looking for immediate results upon his activation, but he now realizes it is not that easy.

"That's what's been tough is to just stay with the process," Mercer said. "And that's what a lot of people have told me -- continue to stay with the process and continue to get your work in and it's eventually going to turn."

Gray lasted 4 2/3 innings and allowed five runs and nine hits, including two homers and two run-scoring doubles, while throwing 81 pitches. Gray is 0-1 with a 2.70 ERA in four road starts and 0-1 with an 8.27 ERA in five starts at Coors Field.

"One thing I really need to work on is setting sights here," Gray said. "I know it it's not going to break as much. Fastballs are not going to have as much sink or run. I really think it just comes from mindset. I think I really need to work on pitch location. That's really going to be a difference-maker pitching at Coors."

NOTES: Pirates INF Jung Ho Kang (left knee surgery) was placed on the 60-day disabled list. The Pirates selected the contract of OF Keon Broxton from Triple-A Indianapolis. Broxton made his major league debut as a pinch runner in the ninth. He stole third base and scored on a throwing error ... Rockies LHP Jorge De La Rosa was scratched from his start Wednesday due to right Achilles tendinitis. RHP Christian Bergman will start in his place. ... Colorado LHP Yohan Flande must throw a bullpen session Wednesday in order to make his scheduled start Friday against the Dodgers. Flande sustained a left patellar tendon contusion when he was struck just below the left knee with a line drive in his Saturday start. ... Rockies C Tom Murphy made his fourth start. Manager Walt Weiss said he intends to alternate Murphy and fellow rookie C Dustin Garneau pretty much on a daily basis. Season-ending injuries to C Michael McKenry and C Nick Hundley resulted in the promotions of Garneau on Aug. 19 and Murphy on Sept. 11 from Triple-A Albuquerque.