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

He tossed three innings Thursday in the Mets’ 5-0 Grapefruit League win over the Astros and gave up no runs, one hit and one walk with 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.

“I feel like I was in a great spot to keep pitching,” Holmes said. “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.

He reached 95 mph in his third inning, though acknowledged he’s still figuring out when not to throw his hardest in order to save himself for later innings.

“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.