Ken Fuchs, head of entertainment, sports and games at Yahoo Inc., emailed players early Monday to apologize for problems with the site and its mobile apps.
"I'm disappointed that we failed all of our fans today," Fuchs wrote in the email. "Our first priority is having the best experience for our users, and today we fell short."
Fuchs said the outage started about one hour before the first games kicked off, which he called "awful timing."
Yahoo, along with sites run by ESPN and CBS Sports, is one of the biggest players in the $800 million-per-year fantasy sports industry, hosting leagues where users pick a team of real-life players and compete based on their individual statistics. A study last year found that 32 million people in the United States and Canada were playing fantasy sports, according to the Fantasy Sports Trade Association. Football is by far the most popular fantasy sport.
Many players play casual leagues just for fun, but others gamble money — sometimes big money.
Yahoo said Monday afternoon on its fantasy football Twitter account that the website was back up, but users were still unable to make lineup changes on mobile apps.
Players enraged about the outage took to Twitter, Facebook and Reddit on Sunday and Monday to express their displeasure, some saying they planned to switch platforms as a result.
"I feel like I lost out a lot," said Bryan Smothers, a 27-year-old small business owner from Eureka, Calif., who said the site let him down during a week crucial toward determining whether his path will lead toward the playoffs or a lost $500 buy-in.
Smothers said he waited until Sunday morning to set his lineup because he wanted to be current on the latest injuries, but the outage meant his lineup became locked in with Patriots running back Stevan Ridley, receiver Brandon Lloyd and Buffalo quarterback Ryan Fitzpatrick all on the bench with lesser options in his lineup — including one player on a bye.
Smothers said he appreciated the apology but thinks it may not be enough for players who invest a lot of time and money into leagues.
"You're talking about millions of people," he said.
Hal Spivack, director of product development at fantasy site Fleaflicker, said players began emailing him and sending him tweets asking to migrate their league to the smaller platform, which currently has hundreds of thousands of players.
Spivack said he sympathizes with Yahoo's problems, as New York-based Fleaflicker had a brief midweek outage during Superstorm Sandy. But he said companies running fantasy sports have to prepare themselves for busy traffic and make sure their servers and sites can withstand high demand.
"Even the smallest of issues do get noticed a lot. There is backlash because ... people have bigger things riding on (their leagues)," Spivack said. "People tend to get very upset."
Paul Charchian, president of the Fantasy Sports Trade Association, said almost every fantasy site has had some kind of failure and the Yahoo outage highlights the complexity and scale of how the games are operated.
He said players should have failsafe options in their leagues, like calling commissioners and opponents to let them know what they want to do if a site goes down.
"I would encourage players to remember that this game wasn't always played online," Charchian said. "It's not the end of the world. They'll fix it."
Oskar Garcia can be reached at http://twitter.com/oskargarcia .