As he rounded the bases after blasting a pinch-hit, three-run homer that left most onlookers staring in disbelief, Yoenis Cespedes admitted surprise at one aspect of the hit he called one of the most thrilling of his career.

He swung at the first pitch.

Yup, swinging at the first pitch was the hard part for Cespedes to grasp about his stunning homer Tuesday that turned a seeming defeat into victory and stirred memories of his high-drama heroics last season. Not the fact he was sidelined for three games with a thigh bruise. Not the fact he was about as warm as an iceberg getting prepped to pinch hit.

“What I was kind of first thinking, I didn’t even know it was a home run and I think something that surprised me and my teammates was that I swung at the first pitch,” said Cespedes, who remained out of the starting lineup for a fifth straight game Wednesday, one night after pulverizing Brandon Finnegan’s straight-as-an-arrow fastball that propelled the Mets’ 4-3 victory at Citi Field.

“I never really swing at the first pitch, but I saw that it was just straight and I decided to go for it so I was just thinking about that,” Cespedes said through an interpreter.

“What surprised me is, like I said, after a couple days of being out that I would swing at the first pitch. It doesn’t surprise me. I know that if I hit the ball well I can hit it out so that part doesn’t surprise me as much.”

Cespedes left the clubhouse Tuesday after his heroics but before addressing them with the media. A Mets spokesman said there was a miscommunication so Cespedes recounted the event before Wednesday’s 5-2 win over the Reds, when he sat again — at his request, according to manager Terry Collins.

To recap: the Mets trailed, 3-0, with two on and one out in the seventh inning. Cespedes, expected to return Friday, had just started taking warm-up swings. He got in “less than 10” when summoned.

“I didn’t expect any of it. I don’t think I even expected to play. I was back in the trainer’s room still getting some treatment and working on that and they came back to find me to say, ‘Terry wants you to go out there,’ ” Cespedes said. “And I started getting ready to work my way out there and then that happens.

“I guess it’s one of the biggest moments of my career so far.”

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The Mets are hoping for lots more. With a Thursday off day, Cespedes, available to pinch hit Wednesday, essentially got two more days to rest his injury.

“I talked to him last night when the game was over and said, ‘Do you really think you’re ready to play?’” Collins said. “And he said, ‘No. One more day.’ With the day off, he thinks he’d be ready by Friday.”

Cespedes admitted he felt “a lot better” thanks to training-room treatments.

“I think Friday I’m going to start playing. I think I’ll be good to go by then,” Cespedes said, admitting a slight concern about sliding which was how he aggravated his injury last week. “I’m still a little concerned because it’s one thing if I slide with my hands. I can do that but if I have to slide with my legs, that’s the side I would slide on.”

Collins had recounted the near laughable drama of getting Cespedes ready to bat. Collins kept hearing “he’s coming” but needed to wait on the star outfielder.

“I was still just getting ready. … It came to me with the scenario of whether [Lucas] Duda would go on or depending what pitcher they brought out, maybe I would go out,” Cespedes said. “I just still wasn’t ready. I was still preparing so once they told me I had to hurry up, throw on my jersey and walk out there.”

And hit a homer on the first pitch he saw.