(Reuters) - Women’s professional soccer has long been a tough sell in the United States but the National Women’s Soccer League (NWSL) is already in uncharted territory as it gets set to open its sixth season this weekend.

The nine-team NWSL has displayed a sort of lasting power that the two failed U.S. professional women’s leagues that came before it were unable to establish but that does not mean it has a secure future.

“We do feel the league has emerged positioned for long-term stability and growth,” NWSL Managing Director of Operations Amanda Duffy told a conference call.

“The quality of our existing ownerships, the quality of our markets, quality of facilities that the teams and players have access to and each of the owners’ commitment to the long-term development of this league are all areas that we feel we have right now or are in a growing position with.”

Americans are always ready to rally around their national team every four years for the Women’s World Cup, but that same soccer fever fails to burn anywhere near as hot outside of the high-profile tournament.

Neither of the NWSL’s predecessors -- Women’s Professional Soccer and the Women’s United Soccer Association -- made it past their third seasons before collapsing.

The NWSL may be the main residence for players on the U.S. women’s national team and already considered the most successful U.S. women’s soccer league but it is actually coming off a tumultuous offseason.

Kansas City, one of the league’s founding members and NWSL champions in both 2014 and 2015, ceased operations last November while the Boston Breakers, another one of the league’s founding members, folded in January.

The expansion Utah Royals begin play this season.

“The sheer fact that we are heading into year six is a good sign,” said Canadian Christine Sinclair, a forward with the reigning champion Portland Thorns.

“All the other women’s leagues in the U.S. didn’t survive past year three so we’ve doubled that.”

The NWSL is supported by the Canadian Soccer Association and United States Soccer Federation and salaries of the league’s 34 allocated players are paid by both federations.

Four of the league’s teams -- Portland, Houston, Orlando City and Utah, -- are also enjoying support through their affiliation with Major League Soccer clubs. North Carolina are affiliated with North Carolina FC of the United Soccer League.

While pairing women’s and men’s clubs is no guarantee of success it does offer a level of financial backing and stability to clubs that perhaps would otherwise be more volatile.

“The league is in a great place, especially the more and more you see teams pairing with MLS franchises,” said Sinclair, who also played in the now-defunct Women’s Professional Soccer league.

“As a team here in Portland we have benefited from that stability, from that fan base, from the organization already being in place.”

Portland, who kick off the 2018 campaign on Saturday against North Carolina in a rematch of last year’s championship final, had an average attendance of more than 17,000 spectators last season, more than triple the league average of 5,083.

Thorns head coach Mark Parsons said the NWSL is “light years” from where it was when he took over the Washington Spirit midway through the 2013 season.

“There is no doubt that this league continues to get stronger, continues to move forward,” said Parsons. “At the same time that’s not an excuse to slow down.”