BOSTON — Mookie Betts was running so hard down the first-base line that he nearly turned the corner without touching the bag.

That’s understandable.

By that point, only two innings into Boston’s home opener, the 22-year-old phenom had already put together a half-hour highlight reel more worthy of a week.

Betts hit a three-run homer, made a sensational home-run-robbing catch and stole two bases for a scintillating start that sparked the Red Sox to a 9-4 win over the Washington Nationals at Fenway Park on Monday afternoon.

“He took the game over,” Boston manager John Farrell said of his center fielder. “All ends of the game, defensively, to baserunning, to at the plate; exciting player, as we’ve seen.”

Rick Porcello allowed four runs (three earned) over eight innings, designated hitter David Ortiz hit his second homer of the season and catcher Sandy Leon recorded his first three hits as a member of the Red Sox, who improved to 5-2.

First baseman Ryan Zimmerman hit his second home run and center fielder Danny Espinosa added his first for the Nationals (2-5), who have lost four of the past five.

But Boston’s leadoff hitter stole the show.

Betts made his major-league debut midway through last season and batted .291 with five homers and 18 RBIs in 52 games.

If Monday was any indication of what’s ahead, it could be a memorable MVP-type of year.

“The way he played today, it was a lot of fun to watch,” said Porcello.

Betts robbed right fielder Bryce Harper of a two-run homer in the top of the first inning, leaping above the short wall in front of Boston’s bullpen to make the catch.

“I don’t really know what I was going through,” Betts said. “I just know I told (right fielder Shane Victorino) at some point I was going to try and catch it.

“As far as jumping and catching it, it just kind of happened in a blur.”

Kind of like his dazzling display of running that was still to come.

With the infield shift employed against the left-handed hitting Ortiz in the bottom of the first, the speedster stole second, sliding headfirst into the bag. He promptly rose to his feet making a mad dash toward third and again dived headfirst into an uncovered base just ahead of the tag by Washington starter Jordan Zimmermann.

Nationals manager Matt Williams unsuccessfully challenged the rulings at both second and third.

“Because we overshift so much ourselves, we’re always talking to our guys about defending it and what to be aware of,” Farrell said. “But with Mookie’s instincts, you would almost think that was predetermined in his mind.”

Betts didn’t decide to keep going until he saw Zimmermann hanging around the mound.

“So I figured I might as well try and go,” he said. “I didn’t think anybody could catch me.”

Ortiz then slapped Zimmermann’s next pitch over the head of left fielder Jayson Werth for a 1-0 lead.

Betts made it 4-0 an inning later with his second homer of the season, belting a 1-1 slider from Zimmermann into the seats above the Green Monster.

“It gave me a little confidence going forward into the season,” Betts said, “but I know I still have some work to go.”

Boston tacked on four more in the third on two hit batsmen, a pair of fly balls that dropped in the sun, an RBI single by Betts and a sacrifice fly from second baseman Dustin Pedroia.

That was more than enough support for Porcello, who earned his first win with Boston since being acquired in the offseason and signing a four-year, $82.5 million contract extension last week.

Porcello (1-1) allowed four hits and one walk, with six strikeouts. He retired 11 of the first 13 batters before Zimmerman’s blast leading off the fifth.

Zimmermann (1-1) lasted just 2 1/3 innings. The right-hander allowed eight runs (seven earned) on nine hits and one walk.

“I was bad today,” Zimmermann said. “It’s not like me to walk a guy and hit two guys. I really didn’t have a feel for the ball and didn’t know where it was going all day.”

NOTES: New England Patriots QB Tom Brady threw the ceremonial first pitch after joining coach Bill Belichick and owners Robert and Jonathan Kraft in carrying the franchise’s four Super Bowl trophies onto the field. … Boston improved to 69-46 in home openers. … The Red Sox activated closer Koji Uehara from the 15-day DL and optioned LHP Tommy Layne to Triple-A Pawtucket. … LF Jayson Werth started for Washington after being activated from the 15-day DL, where he started the season after shoulder surgery. Matt den Dekker was optioned to Triple-A Syracuse.