The Bruins are fourth in the East, four points ahead of the fifth-place New York Rangers, who defeated the Buffalo Sabres 6-3 on Sunday.
Pittsburgh, which concluded a three-game sweep of New Jersey with a 4-2 win Saturday, is 8-1-1 in its past 10 games.
"It was a hard-fought game," Penguins coach Mike Sullivan said. "There wasn't a lot of space out there. You had to fight for every inch on both sides. From that standpoint, I think it had a playoff feel to it. The [game] was low-scoring. It should've been low-scoring. I don't think there were a ton of scoring chances on either side.
"I think it's good for our team to get into these types of games. I said to our guys before the game, 'We've got to find a comfort level in playing in a one-goal, low-scoring game. That's the nature of high-stakes environment hockey.'"
Jeremy Swayman made 28 saves for the Bruins (27-14-6), who have lost two straight following a six-game winning streak. Swayman, a rookie, has allowed more than two goals once in seven NHL starts and less than two in each of his past three.
"I think the biggest thing is taking positives from each day," Swayman said. "I think our leadership does a really good job with that. I think, even when we were down goal, guys were still pushing, ready for that next shift. We're going to take the positives out of this game and move on forward with it."
The teams will play here again Tuesday.
"Today, I didn't mind our work ethic," Boston coach Bruce Cassidy said. "People assume, a 1-0 game, you're not working. Those are good hockey games. Both teams had to work for every inch of ice out there. Trust me, I had the best seat in the house."
Guentzel gave Pittsburgh a 1-0 lead with a one-timer from the left face-off circle off a pass from Sidney Crosby at 4:03 of the third. That ended a six-game goal drought for Guentzel since scoring a hat trick against New Jersey on April 11.
"I thought we were good tonight," Guentzel said. "Games like this are going to be tight-checking. We have to find out ways to come out on top. We have to wait for our chance to get our opportunities."
Guentzel was denied earlier after Bryan Rust passed the puck to the left post. Guentzel tapped it, but Swayman stretched for a toe save at 12:13 of the second period before turning away Rust on a rebound attempt.
The Bruins have been held to less than three goals in four of their past five games against the Penguins.
"It was a playoff-type game. It was tight-checking all over," Boston forward Chris Wagner said. "I think we're probably better at grinding. I don't think we did enough of that tonight. We were getting pucks back to D. Guys were struggling to get to the net. A couple bounces here and there, but we have another shot on Tuesday to redeem ourselves."
NOTES: Bruins first-line center Patrice Bergeron had two shots on goal in 14:55 of ice time after being a game-time decision. He did not play in a 6-4 loss to the Sabres on Friday because of a lower-body injury. … The Penguins got the first power play of the game when Bergeron was called for high-sticking Guentzel at 18:32 of the third period. … Rust's goal streak ended at three games.
