MLS Commissioner Don Garber says he has been pleasantly surprised by the growing popularity of the sport in the US but insist there is plenty of work to do.

The 13-year old league has slowly thrived and grown since its inception and with crowds beginning to get larger and exposure reaching more and more people, exceeding Garber's expectations.

"When I first took this job, I thought it might be just about turning the lights on and all of a sudden soccer would get in a position to explode," Garber said in an interview.

"Over the last decade I've realised there is tremendous potential for this sport but still enormous challenges.

"But I don't believe even in 1999 that I expected that the sport would be as popular as it is today. The games that have taken place over the last few weeks of this summer, I think, are almost unprecedented in American sport.

"We had 93,000 on Saturday night at the Rose Bowl (in Pasadena) for LA Galaxy against Barcelona which is the largest crowd in the United States since the 1994 World Cup. In many ways the sport has grown in ways I never believed it could.

"But we do recognise that for MLS to benefit from the popularity of the sport we really need to work hard at converting all of these soccer fans into being very committed MLS fans - that is a process that is ongoing and I think will continue to take a long time," he added.