Alex Dickerson was one of three Giants to homer at Coors Field on Monday night, but his blast wasn’t enough to make up for two defensive lapses that cost the club in a 7-6 series-opening loss to the Rockies.

The Giants took a 4-1 lead after Dickerson, Mike Yastrzemski and Chadwick Tromp went deep, but the Rockies rallied for five runs in the bottom of the sixth with the help of mental and physical mistakes by San Francisco’s defense, which has committed 13 errors through 11 games this season, second to only the Royals (14).

• Box score

“It's really important that we tighten up our play,” manager Gabe Kapler said. “We play catch, we make the plays because, particularly in one-run games at Coors Field, every out is so critical. Our offense has shown that they can score runs in bunches, so we really do need to keep every advantage that we possibly can.”

Right-hander Johnny Cueto became the second Giants starter to pitch into the sixth inning this season, but he encountered trouble while facing the Rockies a third time through the order. He gave up a leadoff single to Charlie Blackmon before misplacing a fastball to Nolan Arenado, who crushed it out to left field for a two-run shot that cut the Giants’ lead to 4-3. It was the first home run of the season for Arenado, who entered Monday batting .226 over his first eight games.

“Even if he's hitting .120 or whatever, he's a good hitter,” Cueto said in Spanish. “You have to tip your cap to him.”

Cueto departed after allowing three runs (two earned) over five-plus innings and was replaced by lefty reliever Wandy Peralta, who retired Daniel Murphy before inducing a fly ball to right-center field from Ryan McMahon. Dickerson, who made only his second career start in right on Monday, couldn’t secure the ball as it hit off his glove and fell for a triple. Matt Kemp followed with an RBI single to left field to tie the game, 4-4.

“Dating back to last year, it seemed like for the most part, everything I got to I caught,” Dickerson said. “That's normally been a staple of me defensively. Maybe I don't cover all the ground, but I generally catch everything. It was just one of those balls. I got back there and closed [my glove] just a tick too early, and it popped out.”

A single by Chris Owings subsequently put runners on first and second for David Dahl, who knocked in another run with a single through the right side. Dickerson fielded the ball but then overthrew the cut-off man, triggering another defensive breakdown. Peralta was backing up the play, but Tromp vacated home plate while attempting to corral Dickerson’s errant throw, allowing a second run to score.

• A shimmy, a stumble ... then an out for Cueto

“There were a lot of moving parts to the play,” Dickerson said. “Just kind of visually, there was a lot going on. I haven't played a ton of right, so part of that is just getting a little more comfortable with that movement. I think something caught my eye that put me off just enough to where I sailed the throw. It should have just been a lob play to second base. Just sped myself up is really what happened.”

Kapler said he reviewed the play several times after the game and couldn’t definitively say whether Tromp erred in leaving the plate open.

“It's a bit of a jailbreak play,” Kapler said. “In a perfect world, maybe Tromp stays home. He reads that he's got some backup from the pitcher. But I watched that play right after the game. I watched it five times. I'm not sure if everybody does everything -- Tromp stays home, Peralta picks up that baseball, makes a perfect throw to the plate -- that we record the out.”

The decision to start Dickerson in right field was unusual, but Kapler explained before the game that the Giants felt they’d have a stronger defensive alignment with the speedy Steven Duggar in left and Dickerson in right because of the Rockies’ left-handed hitters’ tendency to put the ball in the air to the left side. The move ultimately backfired.

"In hindsight, you question, ‘Was that the right call?’” Kapler said. “Honestly, I bet Dick makes that [catch] 19 out of 20 times. The throw, I bet he makes almost every time. There's no question.

“That's just part of baseball. Sometimes you set them up in a way and it doesn't work out.”

The Giants made it interesting in the ninth, scoring two runs, but Rockies closer Jairo Díaz retired Dickerson on a flyout to end the game.

“I just kind of had one of those innings where two big flukes really cost us,” Dickerson said.