SAN FRANCISCO -- One night after playing their most complete game of the year, the Giants were undone by a series of costly mistakes in a 7-5 loss to the lowly Rockies in 10 innings on Tuesday night at Oracle Park.

Rookie Gregory Santos gave up back-to-back home runs to Ryan McMahon and C.J. Cron in the top of the 10th as the Giants dropped their first game of the year to the Rockies, who finally collected their first road victory of the season.

The Rockies notched only seven hits, but three were homers, including a solo shot from pinch-hitter Garrett Hampson, who took Giants closer Jake McGee deep to briefly give Colorado a 4-3 lead in the top of the ninth. After working 7 1/3 scoreless innings in his first eight appearances of the year, McGee has now allowed home runs in three of his past four outings.

Brandon Crawford launched a game-tying leadoff home run into McCovey Cove in the bottom of the ninth to force extras, but the Rockies quickly retook the lead after McMahon crushed a 92.5 mph slider from Santos out to right-center field for a two-run shot in the 10th. Cron followed by driving a 99.3 mph fastball out to right field for a solo shot that extended Colorado’s lead to 7-4.

Santos, who was making his third big league appearance, has now allowed six runs (five earned) on five hits -- including three homers -- over one inning in his last two outings.

“Santos has some really, really good stuff,” manager Gabe Kapler said. “As we saw, this is a very difficult league. Good heaters get hit, and even good sliders get hit sometimes. In this particular case, I think you have to give the credit to McMahon. He hadn’t seen that slider, but he was able to jump all over it.”

The Giants cut the deficit to two after Buster Posey singled to drive in the automatic runner, Brandon Belt, in the bottom of the 10th, but it wasn’t enough to prevent the club’s third loss of the year at Oracle Park.

Kapler noted that he had no complaints following the Giants’ 12-0 romp over the Rockies on Monday, but a few early mistakes loomed large on Tuesday.

Right-hander Aaron Sanchez allowed only one hit over 4 2/3 innings in his fifth start of the year, but he walked five hitters, two of which scored on Charlie Blackmon’s two-out, two-run double to left field in the first inning.

Sanchez retired the next 10 batters he faced, but he then walked the bases loaded in the fifth and was lifted in favor of lefty reliever José Álvarez, who speared a comebacker off the bat of McMahon and threw to first for the final out of the inning.

The Giants cut the Rockies’ lead to 2-1 in the bottom of the first after Belt singled, advanced to third on a double by Buster Posey and scored on a groundout by Alex Dickerson, but they watched a potential rally fizzle in the fourth inning following a costly baserunning blunder by Mauricio Dubón.

After Crawford and Dubón strung together back-to-back singles to put runners on the corners with two outs, Jason Vosler lined a double to right field to tie the game, 2-2, and collect his first career RBI. Dubón also tried to score from first on the play, but he ran through a stop sign and was called out after colliding with third-base coach Ron Wotus, ending the inning.

“I think Dubón right now is a little caught in between on the bases,” Kapler said. “Sometimes he feels a little overaggressive. Sometimes he feels a little tentative. I think it’s something that we have to keep talking to him about, keeping your head up and running until the play stops you or a coach stops you. In this case, it’s Wo, obviously. It’s a really, really important play in the game and something that we have to keep working on with Dubi.”

Belt briefly put the Giants ahead, 3-2, with a solo shot off the top of the right-field arcade in the fifth, but the Rockies tied it in the sixth. After Álvarez issued a leadoff walk to Cron and gave up a single to Blackmon to put runners on the corners with one out, Dom Nuñez tapped a potential double-play ball up the middle that was fielded by Crawford.

Crawford stepped on second for the forceout, but his throw to first sailed over the head of Belt, allowing Cron to score the tying run on a fielder’s choice.

“I just think that’s a difficult play to control your body on,” Kapler said. “Given Craw’s defense tonight in general, which was outstanding, and the home run, those are the things that I’ll stay focused on. I think that’s a play that Craw can make, but I also think it’s a very difficult one given the body positioning.”

Crawford later redeemed himself with his clutch home run in the ninth and with an outstanding defensive play in the eighth, when he ranged deep into the hole on a grounder by McMahon and threw to Belt in time for the out, but the earlier miscues still ended up haunting the Giants.

“Making outs on the bases is always frustrating,” Crawford said. “I feel like it typically does come back to bite you later on. Stuff like that is definitely frustrating, but there were plenty of things that happened after that. We could have either added on or kept them from scoring runs. That’s baseball. You’re going to make mistakes, and hopefully make up for them later.”