PHOENIX -- Rockies starting pitcher Aaron Cook understood why he had his shortest performance of the season on Friday night, but Todd Helton was confused over why his night was so short.

Either way, the Rockies came up short for the 10th time in 11 games, 6-2, to the Diamondbacks at Chase Field in front of 23,068. The Rockies are 1-7 with two games left on their first road trip following the All-Star break.

Despite falling five games below .500 for the first time this season, the Rockies (45-50) remain 4 1/2 games behind the Padres in the tight National League West standings. However, that type of baseball won't keep the Rockies in it for long. That's especially so if a team in the division turns hot.

That could be the Diamondbacks, who have won 10 of their last 12 and are a game behind the Padres.

Cook (6-8) threw a season-high 111 pitches in a season-low 4 2/3 innings, giving up five runs on 11 hits.

"I think they were sitting on my sinker, and I didn't make adjustments early enough to be able to get them out," Cook said.

Helton, who went 0-for-4, earned a quick ejection after he requested that plate umpire James Hoye defer to the third-base umpire on a called third strike in the seventh. Helton felt he had checked his swing.

"I wasn't cordial or anything, but I've said a lot worse and not gotten run," said Helton, whose only previous ejection occurred during a bench-clearing fight between the Rockies and the Reds in 2000. "I didn't think I was in any danger of getting myself run.

"I thought I'd at least earned enough to say my say ... but it happens."

Way too often the Rockies' innings are ending poorly.

"One of the things that's hurt us the last three weeks is inability to put innings away when we've have to, on offense and on the other side to cash in when we have two outs," Rockies manager Clint Hurdle said. "It was a constant theme in the Cincinnati series and we saw it in Pittsburgh."

Diamondbacks right-hander Claudio Vargas (8-6) forced popups from Helton and Matt Holliday with a runner on second in the first inning, and he fanned Cook and forced a Jamey Carroll grounder to end the second with the bases loaded.

Vargas held the Rockies to six hits and two runs, on a Yorvit Torrealba fielder's-choice grounder in the fourth and Garrett Atkins' 12th home run of the season in the fifth.

"The three-hole hitter is not helping out too much. I know that much," Helton said. "I definitely take my share of the responsibility."

In a repeat of the Rockies' last game, when Jason Jennings gave up all of the runs with two out in a 6-5 loss at Pittsburgh, Cook could not finish innings.

Shawn Green popped his 1,000th career RBI with two down in the first. The D-Backs scored three of their four runs with two down in the third on a Johnny Estrada double, an Orlando Hudson single and a Stephen Drew triple.

In that inning, the Diamondbacks' Luis Gonzalez knocked the 526th double of his career, which put him ahead of Ted Williams and tied Dave Parker for 30th on the all-time list.

Part of the problem was poor strategy, Cook said. The Diamondbacks looked for sinkers on the outside part of the plate. Cook said he should have used four-seam fastballs inside to keep hitters honest. But his he also left his sinker over the plate too often.

"You never know," said Cook, who joined teammates Jennings and Jeff Francis as pitchers with the most losses (eight) despite an ERA below 4.00 (3.88).

"If I get a pitch or two and get out of that third inning and not give up four runs, I might settle down, might not," he said. "It's just one of those nights I didn't have it. It hasn't been often this year."