PITTSBURGH -- Nobody plays more one-run games than the Pittsburgh Pirates.
No wonder they are starting to look so comfortable when things get tight.
Pittsburgh improved to 23-17 in one-run games this season, including 16-8 at home.
"We feel like we're never out of anything, especially in this ballpark," Mercer said. "We just kind of feed off each other and string together some good at bats, and good things happen."
Walker led off the inning with a single against Colorado's Chad Bettis (0-2), moved to second on a sacrifice bunt, and scored one batter later when Mercer hit a drive deep into the gap in left-center.
"He's capable of driving in runs," Pirates manager Clint Hurdle said of Mercer, who is batting .358 in July. "He's getting more confidence every time he takes the field. Nights like tonight, when you can end it like that in a walk-off, just add to that."
"I threw the pitch, it was going to where I wanted it to, and he did an excellent job of getting to it and hitting a line drive. I did not see it," Hughes said. "I feel like it just went right into my glove."
Walker went 3 for 5 with two runs scored for the Pirates, who moved to within 2.5 games of first-place Milwaukee and St. Louis in the crowded NL Central.
Tulowitzki, who leads the majors in batting average (.340) and runs (71), hit a slow roller to shortstop in the fourth against Pittsburgh starter Charlie Morton and grabbed his leg while nearing first base. He walked gingerly to the dugout before being replaced by Josh Rutledge.
"Nothing serious," Tulowitzki said. "We'll see how I wake up [Sunday]. The good news is I don't think I tore anything or did anything to where I'm going to miss an extended period of time."
Colorado starter Brett Anderson, acquired in a trade from Oakland last December following a humbling season in which he went from Opening Day starter to the bullpen, spent seven innings keeping the Pirates off balance with a dizzying array of breaking balls.
The left-hander allowed one run and struck out eight with just one walk, his only flub being consecutive doubles by Walker and Gaby Sanchez in the fourth that put Pittsburgh in front.
He retired the last 12 batters he faced and was in position to earn his first win since April 7, 2013, when Gonzalez took a curveball from Morton in the seventh and sent it streaking into the seats above the Roberto Clemente Wall in right field.
"I think I threw more pitches tonight than I did in two-plus years," Anderson said. "I feel like I battled pretty well and gave us a chance."
For the second straight night, however, Colorado's bullpen faltered.
Pittsburgh scored three times on Friday after Rockies' starter Jorge De La Rosa exited to pull out a 4-2 win. The Pirates put a similar rally together 24 hours later when Anderson left after 110 pitches.
Two walks and a single off Adam Ottavino with one out in the eighth loaded the bases, and Walker laced a single to right against Tommy Kahnle to score Josh Harrison and tie the game. A perfect throw from Gonzalez in right nailed Andrew McCutchen at the plate and kept it even.
The wayward curveball Morton surrendered to Gonzalez marred an otherwise steady outing for the resurgent right-hander. Morton gave up two runs and five hits, striking out three and walking one, a performance that raised his ERA at PNC Park this season to 2.18.
The Pirates held LF Starling Marte out of the lineup. Marte left Friday night's win in the eighth inning as a precaution after getting hit in the head with a fastball by Ottavino. ... Colorado 1B Justin Morneau sat out his second straight game with a stiff neck. Morneau hopes to play Sunday in the series finale. Morneau played 25 games for the Pirates at the end of the 2013 season, batting .260 while helping Pittsburgh to its first playoff berth in 21 years. ... Pittsburgh's Jeff Locke (2-1, 2.89 ERA) faces Colorado's Tyler Matzek (1-4, 4.97) on Sunday.