Bay drove a 2-2 fastball from Chad Qualls (0-1) into the left-field seats with Jack Wilson aboard with one out. Wilson had reached on an infield single.
The improbable comeback struck a chord with the young Pirates. Sure it was only one win, but it was in Houston, where they lost 42 times in 53 previous games, and after being down, 2-0, in the eighth against one of the best pitchers in baseball, on Opening Night, no less.
Bay was clearly pumped as he high-fived teammates and was still excited in the clubhouse when it was all over.
"That's about as much up and down as you're going to get from me," Bay said. "The manner in which we won, we were kind of down and out and then bang, right back in it. We've been in that position a lot of times last year and the year before, only to give it up.
"For this team, it was huge and it meant a lot to me being on this team and kind of being a leader. Just to do it at this point and on Opening Day, it meant a little bit more."
Pirates manager Jim Tracy called it a tremendous start to the season.
"It was a great baseball game, that's all there is to it," Tracy said. "We kept playing and we kept pitching and we kept playing defense."
Nady powered Lidge's first pitch into the Crawford Boxes in left field to tie the game at 2. The Pirates then got a double from Jose Bautista, a walk by pinch-hitter Ryan Doumit and nearly got the go-ahead run home when pinch-hitter Don Kelly -- in his first Major League plate appearance -- blooped a pop fly toward left that Houston shortstop Adam Everett just managed to run down to end the inning.
"I think it helps to drive another point home that we talked about a lot during the course of the spring," Tracy said. "Just maintain our approach and continue to play the game right, don't give at-bats away, show patience at the plate. All those different types of things, it just helps to encourage further the realization that these kids know how to play in games like this."
Pittsburgh starter Zach Duke limited Houston to two runs on eight hits in seven innings, a better road showing than all but two of Duke's 17 road starts last year, and yet he left the game trailing, because Oswalt stifled the Pirates on one run on five hits in 7 2/3 innings.
"Honestly other than the home run that Scott hit, I felt like I was pretty much in control," Duke said. "They got some hits on some good pitches, but I was able to let my defense work for me, and they were outstanding."
Nate McLouth, pinch-hitting for Duke in the eighth, homered off Oswalt to break the shutout, but the Pirates were unable to get anything off the Houston bullpen until Nady's shot.
Duke rolled through the first four innings while holding the Astros to three hits and avoiding the typical initial inning frustrations (31 first-inning runs allowed last year) that dogged him in the past.
Duke's string of matching Oswalt zero for zero ended in the fifth.
With one on and nobody out, Astros right fielder Luke Scott powered a 2-2 pitch an estimated 430 feet into the restaurant seating beyond the center-field fence to give the Astros a 2-0 lead.
The Pirate couldn't mount much of an attack against Oswalt, but Duke was helped by his teammates on the defensive side, as they turned three double plays behind the 23-year-old.
"I don't think there's any better double-play combination out there than [Wilson] and [Jose] Castillo, and [Jose] Bautista plays a very, very good third base," Duke said. "I just feel like I can throw it over the plate, let them hit it into the ground, and good things are going to happen."
The third of those twin killings saved Duke an earned run in the sixth as second baseman Castillo's diving grab of a Carlos Lee liner took away what would have been at least an RBI single for the Astros' $100 million man. Castillo then flipped to Wilson to complete the double play.
That save turned out to be huge as it would leave the Pirates only one run down when Nady came to the plate in the ninth.
"With [Lidge], you've got to try to have a zone, fortunately I got a fastball in and I was able to put some wood on it," Nady said. "With him, he's got good stuff, you've got to battle up there and try to find something you can drive."
There's still 161 games to go, of course, but for one night, the Pirates are riding high. And who knows, maybe this is the type of win that will serve as a stepping stone to better things.
"I think it is [more than just a win] to us, especially with how we've put a lot of pressure on ourselves to get off to a good start," Bay said. "We got off to such a terrible start last year, and it was so hard to come back from that. I think it did mean a little bit more."