RIO DE JANEIRO - Not even the most hopeful of Sepp Blatter's critics could have seen it coming this soon.

On Friday Blatter had been reelected for a fifth straight mandate as the president of world football's governing body FIFA.

When he delivered his cheerful victory speech to FIFA's executive committee, he vowed "age would be no barrier" in his quest to steer "the ship back to the beach" amid a corruption scandal that has shaken the organization to its core.

But the new four-year term barely lasted four days.

Blatter looked every bit his 79 years on Tuesday as he hunched over a prepared statement at FIFA's headquarters in Zurich to announce his resignation from the sport's most senior position.

"While I have a mandate from the membership of FIFA, I do not feel that I have a mandate from the entire world of football - the fans, the players, the clubs, the people who live, breathe and love football as much as we all do at FIFA," Blatter said.

He added that a request for an extraordinary meeting of FIFA's executive committee would be made at "the earliest opportunity" so that a successor could be elected.

Unsurprisingly, the announcement was prime social media fodder. The underlying message that swept Twitter and Facebook news feeds was "goodbye and good riddance" to a man whose popularity among football fans has ebbed to new lows over the past week.

But there were also words of praise; even from one of his most outspoken critics, UEFA president Michel Platini.

"It was a difficult decision, a brave decision, and the right decision," European football's top official said.

Many questioned the timing of the announcement. Why now, just days after he spoke so defiantly about his next term in charge?

The answer could lie in the myriad graft allegations leveled against his colleagues.

Blatter was not named in an indictment by the US Department of Justice last week that accused 14 people - including seven top football officials - of racketeering, wire fraud and money laundering.

He has repeatedly proclaimed his innocence, saying it was impossible for him to monitor the actions of every official from each of FIFA's six confederations.

But it is clear he has felt the heat of developments over the past few days.

None of those developments have been more significant than explosive allegations in the New York Times that FIFA secretary general and Blatter's right-hand man, Jerome Valcke, was involved in wire transfers of bribe money for World Cup bids.

Valcke denied the accusation, as did FIFA in an official statement on Tuesday.

But for Blatter, who has fought off allegations of corruption since taking office in 1998, the claims meant the scandal has now reached his inner circle.

Pressure from FIFA's major sponsors is also likely to have played a part in Blatter's decision.

It took about an hour after Tuesday's announcement for Coca-Cola to issue a statement endorsing the move.

"Our expectation remains that FIFA will continue to act with urgency to take concrete actions to fully address all of the issues that have been raised and win back the trust of all who love the sport of football," the beverage company said.

"We believe this decision will help FIFA transform itself rapidly into a much-needed 21st century structure and institution."

Blatter is also understood to have been urged to step down by his family, especially daughter Corinne.

Whatever the underlying reason for his about-turn, the process to elect his successor will not be so swift.

FIFA statutes require four months' notice for elections to be held, meaning Blatter is likely to remain in the job at least until the end of the year.

It is long enough, he says, to set the wheels in motion for structural change at FIFA.

"Since I shall not be a candidate, and am therefore now free from the constraints that elections inevitably impose, I shall be able to focus on driving far-reaching, fundamental reforms that transcend our previous efforts," he said.

A few months will not allow him to rid FIFA of all its ills. But there is hope that, with his legacy at stake, it might just be long enough for Blatter to write a bright addendum to what is already one of the darkest periods in football's history.