The traffic kept mounting and Steven Matz kept swerving through it without so much as a scratch.

The Mets lefty put 12 runners on base — 10 hits and two walks — in six innings Thursday and lived to tell about it. Nine Nationals reached scoring position against him, but only one scored.

Matz stranded the rest, gutting through his outing and fending off the mental breakdowns he has worked to eliminate from his game before the Mets came through with a 6-4 win over the Nationals at Citi Field.

“This is a huge leap for Matzy, as far as his ability to shrug things off, focus on that in-between pitch routine,” manager Mickey Callaway said. “I could see it on his face tonight. I didn’t really notice one time where he got rattled or wanted to spike a baseball or anything. He stayed under control and understood, ‘I just need to execute pitches,’ because he had some really, really tough situations to get out of against right-handed hitters that have a very huge advantage against lefties. He executed pitch after pitch and stayed calm and got out of all those jams.

“It was an unbelievable outing by him.”

Matz, who received a no-decision, was at his best when he was in trouble, lowering his rotation-best ERA to 3.63. In his second start since returning from the injured list with radial nerve discomfort, he kept the Nationals to 1-for-11 with runners in scoring position.

The only damage came in the sixth inning. Juan Soto led off with a double before Brian Dozier hit a dribbler between first and second. Adeiny Hechavarria tried to shovel the ball to first with his glove, but it flew over Pete Alonso’s head, allowing Soto to score. Dozier was thrown out at third on the play, and even after letting up another single, Matz settled down to strike out the next two batters to end his day.

“I just focused on making a pitch, that’s it,” said Matz, whose 10 hits allowed tied a career high.

Matz did not have any clean innings, but used two double plays and seven strikeouts to help clean up his messes. He leaned on his curveball — the pitch that gave him trouble during his nerve issue — throwing it a season-high 27 times, per Brooks Baseball.

“The curveball was working well for me today so I stuck with that and was able to get out of a lot of traffic,” Matz said. “It’s just what I had going today.”