Baseball has no shortage of unwritten rules, but among the most important is “don’t give the opposing team extra outs.”

The Philadelphia Phillies are a championship contender, but Wednesday they broke that rule and paid for it dearly.

Leading the Red Sox by five runs in the top of the fifth, the Phillies should have gotten out of the inning unscathed when Rob Refsnyder hit a sky high pop-up behind home plate for what would have been the third out of the inning. But Phillies catcher J.T. Realmuto lost the ball in the twilight, allowing it to fall harmlessly to the ground and giving the Red Sox a second chance to capitalize on a bases loaded opportunity.

Within minutes, the Red Sox scored six runs to take the lead.

The Red Sox pulled off a stunning rally and eventually beat the Phillies 9-8 in 11 innings on Wednesday, drawing two bases-loaded walks after the missed pop-up before Romy Gonzalez delivered a grand slam. Then, after Philadelphia twice tied the game in the eighth and 10th innings, Carlos Narvaez came through with a go-ahead two-run home run in the top of the 11th to help Boston pull out its first extra-innings win on the road this season.

In doing so the Red Sox also averted what at one point was looking like a sure three-game sweep.

Prior to the fifth inning rally, the Phillies were in complete command. The top National League contenders had hit four home runs off Red Sox starting pitcher Lucas Giolito in the first four innings, including back-to-back jacks by Kyle Schwarber and Bryce Harper in the bottom of the first that put Philadelphia ahead 3-0 before the Red Sox had recorded an out.

Harper’s home run was the 350th of his career, and Nick Castellanos and Bryson Stott followed up with solo shots in the third and fourth innings, respectively.

Meanwhile, Phillies starter Jesus Luzardo was cruising and didn’t allow hit until Masataka Yoshida came through with a leadoff double to start the fifth. He then walked Ceddanne Rafaela before getting a strikeout and a flyout, but walked Marcelo Mayer to load the bases.

That’s when Refsnyder stepped to the plate, and when Realmuto lost his pop-up in the sky, everything fell apart for the Phillies.

Refsnyder wound up walking to drive in Boston’s first run of the game, and Jarren Duran subsequently drew a four-pitch walk to make it 5-2. At that point Luzardo had walked three straight and four in the inning overall, but Phillies manager Rob Thomson left his starter in to face Gonzalez, who was 0 for his last 20 entering the at bat.

It didn’t work out, as Gonzalez smoked a 1-1 changeup 415 feet into the left field stands.

Once they had the lead the Red Sox looked as if they were primed to close the deal. The club turned a double play to end the sixth and after Trea Turner doubled to put the tying run in scoring position with two outs in the seventh, Aroldis Chapman was summoned to face Schwarber and drew a flyout to end the threat on three pitches.

But after returning for the eighth and making quick work of Harper and Castellanos, Chapman faced Realmuto and the catcher atoned for his earlier miscue by hitting a game-tying solo homer to straightaway center field.

It was the first home run allowed by Chapman since May 7 and just the third he’d surrendered all season.

Garrett Whitlock threw a scoreless ninth to force extras, and once in the 10th Story pulled a curveball down the left field line to score the ghost runner Duran. But the Phillies quickly answered in the bottom of the frame when Turner moved the ghost runner Stott to third on a flyout and Schwarber tied the game at 7-7 with an RBI single off Greg Weissert.

The Red Sox got out of the inning without further incident after Narvaez caught pinch runner Weston Wilson stealing second, and the catcher came through again in the top of the 11th with his go-ahead two-run home run to left field. The Phillies got one run back against Jorge Alcala on a Johan Rojas RBI single, but Brennan Bernardino finished the job by striking out Max Kepler.

Mayer leaves game

Red Sox rookie Marcelo Mayer was removed from the game in the bottom of the fifth, and shortly afterwards the club announced that Mayer was dealing with right wrist discomfort.