DENVER • Chris Nelson was watching when Nick Masset looked down at the ground.

Talk about a heads-up play.

Nelson stole home in the eighth inning to send Colorado to a 6-5 victory over Cincinnati on Thursday afternoon, overcoming a five-run deficit to complete a four-game series sweep and extending their winning streak to a season-best seven games and send the Reds to their fifth loss in a row. The Reds' lead over the Cardinals in the NL Central fell to five games when the Redbirds later beat Atlanta.

With one out and runners on the corners, Nelson broke for home before Masset threw his first pitch to Miguel Olivo. Inexplicably, Masset stepped off the mound, whirled to his left and toward second base.

"He was just looking right at the ground," Nelson said. "He wasn't paying me any attention. (Third baseman Scott) Rolen was playing back. So I just decided I should just go ahead and go. I didn't stop."

Nelson scored easily to give the Rockies a 6-5 lead. He became the first player to take home for his first career steal since Florida's Dan Uggla accomplished the feat on April 21, 2006, against Philadelphia, according to STATS LLC.

The Rockies, who trailed 5-0 after two innings, had called for a suicide squeeze but it never got that far.

"Once I made up my mind I was going, I was going," Nelson said.

Masset said he came set, stepped off the rubber and initially thought Melvin Mora was going to second.

"I turned around and by the time I looked to third base the guy was halfway home and I didn't have a chance to get him," Masset said. "It was a complete mistake."

Reds manager Dusty Baker said he had never lost a game in this fashion.

Troy Tulowitzki started the winning rally with a leadoff drive off Masset for his 18th homer and sixth in the past seven games.

Jason Giambi, who hit a two-run homer in the fourth, followed with a walk and was replaced by Nelson. First baseman Joey Votto then made a critical error on Mora's slow bouncer. He charged the grounder, but his underhand flip to Masset covering first was high and wide.

Nelson went to third on the play, setting up the 15th swipe of home in Rockies history and first since Matt Holliday in 2006.

Huston Street allowed a leadoff single to Ramon Hernandez in the ninth, then struck out the side for his 17th save.

The Rockies have won 10 straight games against the Reds at Coors Field and 19 of their past 22 games overall against Cincinnati.

Matt Belisle recorded two outs in the eighth to get the victory.

Cincinnati got off to a nice start against Jason Hammel, who pitched seven innings.

Brandon Phillips singled, stole second and advanced to third on a wild pitch before scoring on Votto's swinging bunt in the first.

The Reds added four more in the second. Ramon Hernandez hit an RBI double and scored on Paul Janish's single. After Travis Wood sacrificed, Drew Stubbs hit his 16th homer and first since Aug. 14.