Koji Uehara, John Farrell, Christian VazquezBOSTON — There have been very few constants — positive ones, at least — in the Red Sox’s 2014 season. Nothing has been more stable than Boston’s ability to win games when leading after eight innings.

In a year in which new problems continue to arise for the Red Sox, however, closer Koji Uehara became the latest victim of the Baseball Gods’ cruel joke. Uehara surrendered five runs in the ninth inning Friday as the Red Sox fell to the Seattle Mariners 5-3 at Fenway Park.

The Red Sox carried a 3-0 lead into the ninth inning. Yoenis Cespedes knocked in all three of Boston’s runs with a towering home run onto Lansdowne Street off Mariners ace Felix Hernandez in the sixth inning. It looked like the Sox were well on their way to snapping a five-game losing streak when “Sandstorm” blasted out the Fenway sound system, signifying the entrance of one of baseball’s most reliable closers.

Logan Morrison’s one-out single into left field was the first indication that putting away the Mariners — now winners of 12 of their last 16 games — wasn’t going to be a piece of cake. But it was Endy Chavez’s two-out walk that led to everything unraveling.

Chavez battled back from a 1-2 hole to earn a free pass. He saw 10 pitches, even fouling off three straight before Uehara missed with a 3-2 splitter. It was an impressive plate appearance against a pitcher who entered Friday with the best strikeout-to-walk ratio (10.81) among all major league relievers with at least 75 innings pitched since the beginning of 2013.

Uehara quickly jumped ahead of Chris Denorfia, who pinch-hit for Brad Miller. Again, Uehara couldn’t place the final nail in the coffin, though, as Denorfia dropped a single into right field to load the bases.

Austin Jackson, who has given the Mariners a spark since being acquired from the Tigers as part of the trade that sent David Price to Detroit, delivered a huge blow. He doubled to left field to plate Seattle’s first two runs. Jackson, like Chavez and Denorfia, managed to produce with two strikes.

Dustin Ackley didn’t stick around to see a second strike versus Uehara. He lifted an 0-1 fastball into shallow left field. Cespedes charged in as Brock Holt — playing shortstop in place of Xander Bogaerts, who left the game to be evaluated for a concussion after being hit in the head with a pitch — raced out. The ball dropped in for a hit, and Denorfia and Jackson both raced around to give the Mariners a 4-3 lead.

Robinson Cano added an RBI single that sent Uehara to the showers.

“I really didn’t think he struggled a whole lot,” Holt said of Uehara. “He was making pitches and they were blooping balls in here and there. Austin Jackson hit a ball off the wall, but other than that, no one really got very good wood on it. They were just finding holes. That’s something we haven’t been able to do. It seems like the other teams have.”

The Red Sox were 44-0 when leading after eight innings before Friday’s loss. They were the only major league team that had not lost all season when entering the ninth inning with a lead. It speaks to Uehara’s dominance, more than anything, though the heavily worked 39-year-old has allowed at least one run in three straight appearances.

“Baseball’s a crazy game,” said Red Sox starter Joe Kelly, who tossed five shutout innings before exiting the game for precautionary reasons due to a minor tweak in his shoulder. “Koji’s one of the best in the whole entire league. And to see that happen just shows how hard the game of Major League Baseball is. The guy’s been so dominant. For me, looking on the other side of the clubhouse and watching him in the World Series to watching him be on my team this year, it’s definitely a really, really hard game and stuff like that happens.”

Until Friday, “stuff like that” hadn’t happened to the Red Sox, even amid their chaotic World Series defense. But times are tough in Boston and no one is immune to imploding.