SAN FRANCISCO — Opportunity wasted.

The Mets were sitting pretty after stomping the Giants in consecutive games, but a combination of ineffective starting pitching and mediocre offensive production over two days conspired to prevent a series victory.

Tylor Megill followed an ugly David Peterson start a day earlier with his own clunker on Sunday and the Mets stalled offensively in a 5-4 loss to the Giants at Oracle Park.

It left the Mets with a series split against a reeling NL West team.

Drew Smith surrendered a double to Mike Yastrzemski in the eighth inning that allowed the go-ahead run.

Smith’s walk to Joc Pederson started that rally.

The Mets finished 7-3 on a West Coast trip that included visits to the Athletics and Dodgers, but they faced a cross-country flight home on a two-game skid.

“We want to win the close ones, and the close ones hurt a little bit more,” Mark Canha said. “It was just one of those weird games where it just kind of fell into their lap.”

Buck Showalter did not use Adam Ottavino or David Robertson in the series, but said Ottavino would have pitched the ninth on Sunday.

The manager indicated Smith’s stuff and effectiveness suggested he was worthy of entering a tie game in the eighth with Ottavino and Robertson in the bullpen.

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“We’re comfortable with any of those guys pitching,” Showalter said. “We’ve got multiple people that we feel very much the same about, and we’re comfortable with any of them.”

Megill scuffled through four innings in which he allowed four earned runs on six hits with one walk and two strikeouts.

The right-hander departed after 81 pitches in what was his shortest and least-effective start of the season.

“Not great,” Megill said of his performance. “I was falling behind early, and they were coming out real aggressive. … I am just pretty disappointed with today’s start.”

Jeff Brigham gave the Mets two perfect innings with four strikeouts. The right-hander has not allowed a base runner in his four appearances since his recall from Triple-A Syracuse. Brooks Raley escaped trouble in the seventh by striking out Michael Conforto after LaMonte Wade Jr. smacked a two-out double.

Pederson, activated from the injured list before the game, stroked an RBI single in the first that put the Mets in a 1-0 hole.

Megill had gotten J.D. Davis to hit into a double play after allowing a leadoff single to Wade Jr. and walk to Conforto before Pederson delivered.

Thairo Estrada’s homer leading off the second extended the Mets’ deficit to 2-0.

The blast was the fourth allowed by Megill in his last three starts.

The Mets ignited a two-out rally in the third to get on the board against Ross Stripling.

After Starling Marte singled, Francisco Lindor slashed an RBI double to left-center, but was left stranded when Pete Alonso was retired.

Conforto’s dropped fly ball gifted the Mets a run in the fourth, when they scored twice to go ahead 3-2.

Stripling loaded the bases with nobody out before Francisco Alvarez struck out and left-hander Taylor Rogers got the pinch-hitter Canha to fly to Conforto, whose drop in right field was ruled a sacrifice fly and error. Brandon Nimmo followed with a sacrifice fly — that otherwise would have been the third out — for the inning’s second run.

Megill slogged through a fourth inning in which he allowed singles to Yastrzemski, Brandon Crawford and Blake Sabol, the last of which drove in a run. Megill’s error on a pickoff throw to first base allowed Crawford to score, placing the Mets in a 4-3 hole.

In the sixth, Alvarez tied it 4-4 with his first homer since his recall from Triple-A Syracuse on April 7. The rookie catcher doubled his RBI total with one swing.

Canha said the positives of the trip outweighed the sting of how it finished.

“There’s a lot of good things that happened on this trip when I look at Jeff Brigham, Alvarez got to play a lot, [Brett] Baty got to play a lot,” Canha said. “It’s going to improve our depth and help us in the long run to have guys be in these games and be active in these games, playing a significant role and it’s going to pay significant dividends, I think.”