SAN FRANCISCO >> A sun-kissed sellout crowd celebrated more than just another Giants win Sunday at Oracle Park.

This 5-4 victory came in walk-off fashion, it secured a sweep of their home-opening series against the Seattle Mariners, and it extended baseball’s longest active win streak to seven.

The Giants, after sweeping their previous three-game series in Houston, own an 8-1 record for just the second time since moving to San Francisco in 1958; the 2003 club opened 13-1.

Pinch-hitter Wilmer Flores delivered a two-out single in the ninth to score Luis Matos, one pitch after a phenomenal catch injured Mariners right fielder Victor Robles.

“We have a feeling where we can win the game no matter what,” Giants manager Bob Melvin said. “When you lose the lead in the ninth, it’s a little demoralizing. To be able to come back again, it shows what we’ve developed here, a certain identity that we can win those close games, we can win any kind of game.

“If we have an opportunity to win in the ninth or late, we have a good feeling it’s going to happen,” Melvin added, “especially in this ballpark with fans going crazy.”

Flores’ game-winner was extra stunning considering the macabre scene it followed. Robles daringly snared Patrick Bailey’s flyball for the Giants’ second out; Matos tagged up and advanced from first to third on the play, but a replay review moved Matos back to second base.

Robles had to be carted off the field after his left arm collided with the waist-high wall and foul-ball netting.

“He took on a wall we’ve never seen anyone take on in this park’s 25-year history,” Mike Krukow said on NBC Sports Bay Area’s broadcast. “It was one of the most spectacular catches we’ve seen in this ballpark.”

While Flores said he had to block out that “unfortunate” scene, his goal as a pinch-hitter is always “to try not to be a hero and stay in the moment.”

By making a concerted effort to not pull Gregory Santos’ 98 mph sinker, Flores heroically delivered his ninth career walk-off hit.

Said Melvin: “That’s the guy you want in that situation. He knows what to expect, he knows how to handle it and wasn’t trying to do too much. The hole was open between first and second (base). He’s done it so often.”

Once Flores’ hit headed for greener pastures, “it was great, I knew I was going to score,” Matos said.

Mike Yastrzemski, whose three-run homer staked the Giants to a 4-2 lead in the fourth inning, got the game-winning rally started with a leadoff, four-pitch walk. Matos then reached on a fielder’s choice, with Yastrzemski’s slide into second base breaking up a potential double play.

Camilo Doval entered in the ninth seeking his third save of the season, and he was one strike shy of doing that. But clean-up hitter Randy Arozarena lined a two-out, two-strike single over a leaping Matt Chapman at third, driving in Robles for the tying run. Doval escaped an ensuing bases-loaded jam by getting Mitch Garver to pop out to Chapman.

Yastrzemski’s three-run homer came on an opposite-field approach that earlier delivered singles by Jung Hoo Lee and Heliot Ramos, the latter of whom drove in Willy Adames for the Giants’ first run of the day.

“I got beat by fastballs three times in my first at-bat (a strikeout), so that time I told myself I wasn’t going to get beat,” Yastrzemski said. “Shooting the ball the other way is all you need in that situation and I squared it up.”

Yastrzemski’s homer was his first of the season, and it fulfilled a pregame request by his daughter, who also asked for a triple and a splash hit. The timely, go-ahead shot came on a belt-high, first-pitch fastball by Mariners starter Bryan Woo, an Oakland native who threw a first-pitch strike 72.9% of the time last season for MLB’s best mark since such tracking began in 1988.

Flores has reached base in all nine of the Giants’ games, but he was out of the lineup up until the ninth inning.

“I can’t run Flo out every single day. I know it’s just DH-ing and we’re getting a lot of production out of him. I want to keep him healthy,” Melvin said pregame. “This was a good day to get Matos in there, get (Heliot) Ramos off his feet and give Flo a day off, though I still have him coming off the bench.”

Indeed he did.

After pitching six scoreless innings last week to win in his native Houston, Jordan Hicks allowed a pair of solo home runs Sunday, with Nos. 2 and 3 batters Julio Rodriguez and Cal Raleigh going deep in the first and third innings, respectively. Hicks got pulled after 90 pitches — with one out in the sixth, two men on base, and a standing ovation from a sellout crowd in his first home start of the season.

Reliever Randy Rodriguez promptly yielded an RBI single to Ryan Bliss that trimmed the Giants’ lead to 4-3. Rodriguez failed to back up Matos’ throw home that bounced past catcher Patrick Bailey, and not only did that allow two Mariners to advance a base, Matos was charged with an error — only the Giants’ second all season. Matos subsequently made a rally-ending catch near the third-base stands to preserve the 4-3 lead.

Seattle threatened to spoil things in the eighth with a pair of pinch-hit singles off Erik Miller, but Matt Chapman started a 5-4-3 doubleplay to end the threat. Another web gem came earlier from the infield’s left side when Adames, in the fifth inning, tracked down Julio Rodriguez’s grounder in shallow center field and rifled a one-hop bullet in time to first baseman LaMonte Wade.

NEXT UP >> The Giants next host the Cincinnati Reds in a rematch of last week’s season-opening series, with today’s starters the same as Game No. 1 in Logan Webb vs. Hunter Greene.