Milwaukee reliever Eric Gagne, recently summoned off the disabled list, got out of the top of the ninth with no damage.
Yates, one of a handful of candidates that may potentially take the reins of the closer's role in Matt Capps' absence, quickly got ahead of the leadoff-hitting Weeks, 0-2.
Yates thought he had Weeks swinging on a 1-2 slider. First-base umpire Charlie Reliford, however, didn't ring Weeks up. On the next pitch, Yates located his fastball on the outside corner. Again, he wouldn't get the call.
"I thought we had him struck out twice," he said. "I looked at the video and saw the check swing, and it looked like he went. But that's over and done with. I've got to make a better pitch to him on a 3-2 count and get him to swing."
That 3-2 pitch was low, and Weeks trotted down to first with a leadoff walk.
"Late in the games, you don't put the man on," Yates added. "You make them earn it. You make them swing the bat. And I didn't do that. I gave them a free base."
A sacrifice bunt moved Weeks up 90 feet before manager John Russell opted to walk the hot-hitting Ryan Braun, bringing Fielder to the plate with a double play possibility.
Despite the fact that the left-handed-hitting Fielder came into the game batting much better against righties than lefties (.288 vs. .218), Russell stuck with Yates.
"He's throwing the ball great," said Russell, who has shown before not to be a firm believer in following matchup odds. "He's the best chance for a double play. You start running three or four bullpen guys in one inning and try to get out of it, it's going to hurt us."
Yates got ahead of Fielder, 0-2. But Fielder soon took the fourth pitch of the at-bat and dumped it into no-man's land in left-center to put the Pirates on the wrong end of a walk-off victory for the second time on this road trip.
The loss overshadows another outstanding start from Maholm. Backed up by a supporting cast that played solid defense, Maholm struck out six in an eight-inning outing that screamed efficiency.
The lefty needed only 102 pitches to finish eight innings, and has now pitched through at least the eighth four different times this season. No other pitcher on Pittsburgh's staff has done so once.
"He's kind of that stabilizer for the staff," Russell said. "He's the guy who's going to go out there and you know he's going to give you a quality start."
Maholm gave up a second-inning solo homer that gave the Brewers an early 1-0 lead but, from there, he would allow only one baserunner -- infielder Bill Hall -- to advance to second. Hall would be quickly sent back to the dugout after being caught in a rundown between second and third in what turned into a critical eighth-inning double play for the Bucs.
"I think that if you're attacking the zone, keeping the ball down and letting them hit some early groundballs ... you'll be able to go deep in games," said Maholm, who lowered his ERA to 4.05 on Saturday. "It worked out."
But Brewers starter Dave Bush was just as good. The Pirates had only four hits off the Milwaukee righty in Bush's eight innings of work, though they strung two of those together in the fifth to finally dent the scoreboard.
After Adam LaRoche doubled to start the inning, Jose Bautista sent Bush's offering over the head of Brewers center fielder Mike Cameron. LaRoche scored on the double, and Bautista advanced to third when the relay throw went past catcher Jason Kendall.
However, with three opportunities to drive in Bautista from third, the Pirates hitters were unable to come through.
Maholm struck out. Jack Wilson hit a weak grounder to a drawn-in infield. And Nate McLouth flew out to end the inning.
"You have to give Bush a lot of credit," Russell said. "We had the one chance. We would like to have gotten another run there. But you have to give a lot of credit to their guy. He threw a really, really good game.
"Maholm threw the ball outstanding. You wish he could get better out of it."