Arenado won games in the ninth with a homer against the Padres on June 2 and a single against the Giants on June 7.
On Sunday, with two on and two out in the seventh and the game tied at 2, Arenado singled to center off Cubs starter Edwin Jackson (6-11) to drive in Carlos Gonzalez from second. Michael Cuddyer scored when Cubs center fielder Junior Lake's throw home sailed over catcher Welington Castillo, hit the backstop and caromed into the Rockies' dugout for a 4-2 lead.
"I should probably handle those situations like I handle every at-bat -- I just try to look for something over the plate," Arenado said. "I was just trying to look for something up, and fortunately I got something up."
Rockies manager Walt Weiss said, "Nolan doesn't shy away from those pressure at-bats. He likes to be the guy. He got a big hit today."
Starting pitcher Tyler Chatwood (6-3) gave up two runs (one earned) in six innings, then sat back as the bullpen handled three eventful but ultimately successful innings. Rex Brothers gave up a hit and a run in the ninth, held his breath as a Cody Ransom moon shot went foul with two outs, then struck out Lake to complete his fifth save.
The Rockies (48-51) remained 3 1/2 games behind the National League West-leading D-backs, who are a half-game ahead of the second-place Dodgers. The Rockies took two of three over the Cubs in the first series out of the All-Star break to start a 10-game homestand. The Marlins arrive Monday to start a four-gamer, followed by three against the Brewers.
Letting a game or a series escape against the Cubs and Jackson -- who entered with a 15.12 career ERA against the Rockies -- would have been damaging.
Before the All-Star break, at the end of a period of multiple injuries to key players, they went 7-9 against the NL West. It wasn't an optimal performance, but the Rockies remained in contention.
"That feeling came into play then, and I don't think it's going to leave us from here on out," said reliever Matt Belisle, who pitched through a two-on situation in a scoreless eighth. "We have a very good knack for understanding the situation but not over-heightening the game, just rising to the occasion to compete. The guys take pride in it."
In the Rockies' first, Charlie Blackmon doubled, Gonzalez delivered an RBI single and Cuddyer added a run-scoring double before Jackson found his groove.
Chatwood threw 21 pitches in the first, which ended with him striking out Lake with the bases loaded. After that, the Cubs didn't threaten until the fifth, when Chatwood walked two before getting Starlin Castro to hit into an inning-ending line drive.
The Cubs' Anthony Rizzo homered to open the sixth -- just the third home run Chatwood has allowed in 13 starts -- as the right-hander labored through the inning. Schierholtz and Lake singled with one out, and Darwin Barney delivered a sacrifice fly to tie the game at 2. After intentionally walking Castillo to load the bases, Chatwood ended his day on a groundout from Jackson.
"I had some traffic out there, especially in the first when they had bases loaded and was able to get out of it," Chatwood said. "It was big going forward, and in the sixth I had some more traffic, but I was able to minimize it."
Jackson's only two walks came before Arenado's key single. Gonzalez drew a walk and stole second, and Jackson pitched carefully to Cuddyer.
"I don't feel like those two walks there necessarily killed me," Jackson said. "You want to come after those guys, but it was two great hitters right there. You definitely want to try to make your pitch in that situation, just one of those things."
Cubs manager Dale Sveum wished Lake hadn't have tried to throw home on the fly.
"We could have bounced it a couple times, I think, and would have caught him," Sveum said.
Arenado sprinted around third but umpires sent him back to second. They ruled that Arenado had not reached first by the time Lake released his throw. By rule, a runner receives two bases on a throw from the field that goes out of play. It ended up not mattering when Yorvit Torrealba grounded out.
A two-run lead proved just enough.
Rockies left-handed reliever Josh Outman gave up a hit and a walk in the seventh, but righty Edgmer Escalona induced a foul popup from Alfonso Soriano and flyout to left from Schierholtz.
After walking Barney with one out and giving up a no-man's-land infield single to Dioner Navarro in the eighth, Belisle got Luis Valbuena to hit a harmless infield grounder.
Brothers earned his first four saves while regular closer Rafael Betancourt was hampered by a right groin injury earlier this season. Sunday was Brothers' first save chance since Betancourt was placed on the 15-day disabled list with appendicitis.
Brothers yielded Castro's leadoff double in the ninth and watched him score on two grounders, but the biggest scare was Ransom's foul ball.
"At first it was in question, then I saw it hooking," Brothers said. "Especially right out of the gate, you want to get things rolling in the right direction."