PITTSBURGH — The Pirates’ comeback ended when rookie Mike Crotta lost control.

Crotta walked three of the four batters he faced in the seventh, including a pass to Seth Smith with the bases loaded to force in the go-ahead run in the Colorado Rockies’ 6-5 win over Pittsburgh on Sunday.

The loss capped a 1-3 opening home series for the Pirates, dropping them to 5-5 after beginning the season by winning four of six on the road — a start that had Pittsburgh fans hoping this squad could finally end a string of 18 straight losing seasons.

“Tough day,” said Crotta (0-1), who was charged with the first run of his career. “I just didn’t execute my pitches; that’s my job. You come in and got to get outs and it’s tough to get outs when you walk people.”

James McDonald settled in after allowing six of the first eight batters to reach, allowing the Pirates to score five unanswered runs before they went hitless over the final five innings.

All four batters who faced Crotta reached base after he relieved McDonald with two outs and a runner on with Pittsburgh clinging to a 5-4 lead in the seventh. Jason Giambi’s RBI single tied the game.

Pittsburgh had only one baserunner after the end of the fourth inning — a Jose Tabata seventh-inning walk. Tabata stole second, but cleanup hitter Lyle Overbay struck out looking to end the threat.

“We just got into a situation where we didn’t continue hitting,” Overbay said. “We didn’t get that killer instinct and get that (big hit).”

The Pirates scored 14 runs in the equivalent of 4Z\x games — including a 14-inning victory Friday — on 28 hits in 41 innings.

“Can we improve on offense? Yes, we can,” manager Clint Hurdle said.

Colorado’s bullpen was nearly flawless and the Rockies improved to 6-2 — their second-best start in franchise history.