Cook pitched eight strong innings and Fuentes, back as the team's closer, earned his first save in 10 months to help the Rockies end a four-game skid with a 4-2 win over the Chicago Cubs on Thursday.
Cook (3-1) won his third straight start and halted the Cubs' six-game win streak. Chicago had won nine of its last 10 games.
"A big-time performance from Aaron, and to finish off like the way we did with Brian was good to see," Rockies manager Clint Hurdle said.
Fuentes, who took over the closer's role from struggling Manuel Corpas before the game, worked a perfect ninth to record his first save since last June 21. He ended a personal streak of seven straight blown save opportunities.
"As much as I tried to make it the same, it wasn't," Fuentes said. "There was so much electricity in the air, the crowd was into it and I just kind of go with my emotions."
Fuentes' performance ended another skid. The Colorado bullpen had surrendered late-inning leads in the last four games, including two straight blown saves by Corpas.
The Rockies broke a 2-all tie in the eighth. Matt Holliday led off with a double and moved to third on a ground out by Garrett Atkins. After an intentional walk to Brad Hawpe, Troy Tulowitzki hit a grounder to Ronny Cedeno at second, but shortstop Ryan Theriot overran second and couldn't make a play and Holliday scored.
"I tried to rush it. If we don't turn it, they score," Theriot said. "I wasn't concerned as much about the baserunner as I was getting the out at first. Troy runs hard and it was on a check swing."
One out later, Ryan Spilborghs drove in Hawpe with a single to left.
Chris Iannetta hit his first homer of the year leading off the seventh to give Colorado a 2-1 lead, but the Cubs came back in the eighth. Geovany Soto led off with a single, Felix Pie and pinch-hitter Daryle Ward followed with walks, and Mike Fontenot drove in pinch-runner Cendeno with a groundout.
Cook escaped with minimal damage when Theriot lined into a double play to end the inning.
"I knew I just had to settle down, make a couple good pitches and fortunately we were able to get out of there with just one run," Cook said.
Hurdle said he didn't consider taking Cook out of the game in the eighth.
"We needed a shot in the arm from a starter. He gave it to us," Hurdle said. "He was the best guy, the best option for me available in the eighth inning."
The Cubs knew they missed a chance to take control.
"We had the bases loaded in the eighth with no outs and come out of it with one run," Chicago manager Lou Pinella said.
Cook settled down after giving up the run, retiring the next 16 batters, including a five-pitch fourth inning. That gave Colorado's busy bullpen some much-needed rest.
Marquis kept pace with Cook. After Colorado got two on in the second inning, he gave up one more hit through the sixth and didn't let a runner get past first base. He allowed a leadoff walk to Holliday in the sixth but retired the next three batters before giving up Iannetta's solo homer to start the seventh.
The Rockies acquired minor-league pitcher Sean Smith from the Indians to complete the trade that sent infielder Jamey Carroll to Cleveland on Dec. 8, 2007. ... Chicago's 15-6 record matched its best start in franchise history. The last time the Cubs won 15 of their first 21 was 1975.