At the start of the baseball season, Sea Dogs fans heard about two prominent prospects earmarked for Portland.

One of them, first baseman Triston Casas, has certainly lived up to his status as the top minor leaguer in the Red Sox system. He helped Team USA win a silver medal in the Tokyo Olympics and had two hits Friday night in a 5-1 victory over the visiting Binghamton Rumble Ponies to raise his batting average to .286.

The other ballyhooed prospect, outfielder Jeisson Rosario, has numbers that aren’t as gaudy. He entered Friday’s game with a .237 average and is slotted eighth in the batting order. And yet, when runners get in scoring position, something clicks with Rosario.

A crowd of 6,202 at Hadlock Field saw Rosario come up in the seventh inning of a close game and deliver a two-out, two-strike single to the opposite field that scored two insurance runs.

“I tried to make contact,” he said, “just put the ball in play.”

Earlier in the game, Rosario used his formidable speed to score from second base on a mishandled ground ball that never left the infield, giving the Sea Dogs a 3-1 lead. His clutch hit in the seventh made it 5-1 and improved his batting average with runners in scoring position to a robust .393, best on the team.

“You saw him foul off some tough pitches,” Sea Dogs Manager Corey Wimberly said. “He continues to work and continues to get better. I’m happy where he’s at right now.”

Another highlight of Friday’s game was the pitching of 21-year-old right-hander Victor Santos, who came to the Red Sox organization last month as the player to be named later in a January trade with the Phillies involving former Sea Dogs shortstop C.J. Chatham.

Santos joined the Sea Dogs after pitching against them for the Reading Fightin Phils. After one relief appearance, he has made four starts, all of them impressive, all resulting in Sea Dogs victories.

On Friday night, he scattered five hits in five innings, allowed only one run (a solo homer by Carlos Rincon to open the fourth), walked one and struck out five. Over his four starts, he has walked only three batters in 23 innings.

His fastball tops out at 94 miles per hour. He pounds the zone, doesn’t waste time, and keeps his fielders on their toes.

“He pitches with great tempo,” said Casas, who faced Santos in Reading and has played behind him in consecutive starts. “He controls his change-up, which is one of his best pitches, and he’s got a pretty sneaky fastball. We feel pretty confident every time he takes the mound that we’re going to get a win.”

Left-hander Rio Gomez followed Santos and tossed a pair of hitless innings, marking his 11th consecutive scoreless appearance. That stretch includes 15 2/3 innings, with nine hits allowed and 15 strikeouts.

The Sea Dogs never trailed. They took a 1-0 lead in the third on a slow chopper by Hudson Potts after Casas singled and Ronaldo Hernandez doubled.

They made it 3-1 in the fourth when a rushed attempt at a double play on a one-out grounder by Grant Williams with the bases loaded resulted in two errors charged to Rumble Ponies second baseman Yoel Romero. Roldani Baldwin singled and Rosario doubled to get things started.

Casas, Baldwin and Rosario each had two of Portland’s nine hits.

The Sea Dogs and Rumble Ponies continue their six-game series with a seven-inning doubleheader Saturday at Hadlock, starting at 5 p.m.

NOTES: David Dombrowski, while he was running the Red Sox, selected Chatham in the second round of the 2016 draft. As president of baseball operations now for the Phillies, Dombrowski views the versatile Chatham as a possible big-league utility player.

A fractured left hand in early June sidelined Chatham for much of the summer, but he returned to Triple-A Lehigh Valley last weekend and has been playing regularly.