For nearly nine seasons, the Red Sox have sold out every home game, the longest streak in major league history and a testament to a fervent fan base, iconic ballpark, and the glow of two World Series titles.

But following the team’s collapse last September and poor start to this season, ticket sales have slowed, putting the streak in jeopardy and curbing prices in the resale market.

Even as Fenway Park celebrates its centennial season, plenty of seats remain for many games this season, a clear step down from the frenzied demand of a few years ago. On resale sites, reasonably priced seats remain for even Friday’s home opener, a perennially tough ticket.

“Prices are definitely down a little from last year,’’ said Jim Holzman, president of Ace Ticket, a ticket reseller that buys most of its supply from season ticket holders. “You don’t have the excitement there was.’’

The Red Sox consider a game a sellout when paid and complimentary tickets equal capacity, so the streak could continue even if paid attendance falls slightly short. Capacity is 37,495 for night games and 37,067 for day games, when the team closes a section of the center-field bleachers to improve visibility for batters.

The Red Sox have sold 2.66 million tickets so far this season, roughly 2 percent off last year’s pace. Sam Kennedy, the Red Sox chief operating officer, said the front office is watching sales closely, particularly with the team stumbling out of the gate, but remains confident the first slate of home games will sell out.

“Anytime there’s a dip we take a close look,’’ he said. “But we are cautiously optimistic. We have a very resilient fan base.’’

Attendance has topped 3 million the past four seasons, but even at the height of demand in 2007-2008, the team would have sold only about 2.7 million tickets by this point in the season, Kennedy said. The remainder are sold throughout the season, with several hundred set aside for each game day.

“We never sell out until the day of the game,’’ Kennedy said.

Fenway Park is among the league’s smallest ballparks, and tickets for big games, particularly on summer weekends and holidays, are usually gone in a flash, sellers say.

But landing seats for early-season or midweek games is far less competitive, even when the team is playing well.

“There are great values to be had,’’ Holzman said.

The drop-off in sales signals that the runaway popularity that sustained the sellout streak since 2003, a span that saw the Sox make the playoffs six times, is tapering off.

“In a way that’s a product of success,’’ said Glenn Stout, a Red Sox historian and author of “Red Sox Century.’’ “Since they’ve won a couple of times in the past decade, there isn’t the urgency anymore.’’

“It hasn’t been a hard ticket to get for the last couple of years,’’ he said. “There hasn’t been that vast clamor.’’

League-wide, attendance has dropped sharply since the recession, falling from 79.5 million in 2007 to 73.4 million last year.

“You don’t have to watch games for long to see lots of empty seats,’’ said Craig Depken, an economist at the University of North Carolina Charlotte who studies professional sports. “People have less disposable income than they used to.’’

Andrew Zimbalist, a sports economist at Smith College, said he was surprised that ticket sales had not dropped more, considering the team’s poor showing at the end of last season and failure to make a deep playoff run in several seasons.

“To me, it’s remarkable they haven’t dropped off more,’ he said. “It looks like they are in pretty good shape, but the real proof will be in how the team holds up.’’

Stout said he believes the sellout streak will continue this season, with sales propped up by the centennial fanfare around Fenway, the league’s oldest park and a baseball mecca.

“It’s difficult to overestimate the pull Fenway Park has for people across the country,’’ he said. “There are more fans of Fenway Park than there are Red Sox fans.’’

Kennedy said team officials consider the sellouts a “huge competitive advantage,’’ but recognizes the streak is fragile, and has to come to an end eventually.

Holzman said the streak’s fate may come down to weather. Nothing curbs demand faster than a cold, drizzly night, particularly during the middle of the week.

“We’ve all gotten spoiled,’’ he said with a chuckle.

Others point to Cleveland as a cautionary tale. With excellent teams and a new ballpark, the Indians sold out 455 straight games from 1995 to 2001. But when the streak ended, interest never fully rebounded.

“As soon as they didn’t sell out, they never sold out again,’’ Depken said.

Peter Schworm can be reached at schworm@globe. Follow him on Twitter @globepete.

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