PORT ST. LUCIE — Through two outings, Clay Holmes’ transition from closer to starter has been nearly flawless.

In the Mets’ 5-0 Grapefruit League win over the Astros on Thursday, he pitched three innings and conceded no runs, yielding just one hit and one walk while recording two strikeouts.

He increased his workload, throwing 42 pitches compared to 34 in his first start as he continues to build up his endurance.

He threw 17-18 more pitches in the bullpen afterward.

Holmes has pitched six innings and given up just one hit and one walk with five strikeouts in two starts this spring.

Reflecting on his performance, Holmes expressed his satisfaction by stating, “I feel like I was in a great spot to keep pitching. Even after I finished there, I feel a lot better than I did last time when I finished in the bullpen. I can see that I continue to adjust to the workload. I really feel like things are trending in the right direction.”

Thursday’s showing marked a significant step in that process, as well.

His 42 pitches included 20 in the first inning as he worked around a Christian Walker double that really was just a bloop to right field.

As a closer, Holmes rarely came back out after innings in which he threw a lot of pitches.

As a starter, that will be required.

And Holmes fared well after the heavy first inning, recording a 1-2-3 second inning and retiring six of the next seven Astros batters.

During his third inning, he hit a speed of 95 mph, although he admitted that he is still learning when to ease off on the velocity to conserve his energy for later stages of the game.

“It’s a very game-like situation, where I have a long first inning, throw [20] pitches and you got 60 pitches to get five or six innings,” Holmes said. “And just feeling that on your body, it’s new. Feeling that taxing inning and then having to do it time and time again, it’s definitely good to experience.”

Brandon Sproat, the Mets’ top pitching prospect, will start Friday’s game against the Nationals.

It’ll be his first outing this spring.

The Mets signed José Ureña as a non-roster invite for potential rotation depth, The Post’s Jon Heyman reported.

Ureña was with the Rangers last year and recorded a 3.80 ERA.

He spent the first six years of his career with the Marlins before stints with the Tigers, Brewers, Rockies and White Sox. He had largely been a starter previously, though was used more as a reliever last year.

Griffin Canning, an option to earn a spot in the rotation due to Sean Manaea’s and Frankie Montas’ injuries, made his first outing of the spring and tossed 1 ²/₃ scoreless innings with two strikeouts.

Brett Baty hit his second home run of the spring Thursday and is 6-for-11 at the plate.