There are derbies and there are rivalries. Most derbies are rivalries, but not all rivalries are derbies. Some in Major League Soccer circles are often guilty of conflating the two. The so-called 401 Derby between the Montreal Impact and Toronto FC, for instance, isn’t really a derby. Neither is the contest between the Seattle Sounders and Toronto FC given that there is over 2,500 miles between the two clubs. And yet circumstances have certainly made them rivals.

Indeed, this Sunday will mark the third time in four years that Seattle and TFC have met in the MLS Cup final. While most of the focus in MLS these past two seasons has been on Atlanta United and Los Angeles FC, two shiny new franchises who have set new standards across the board, the Seattle Sounders v Toronto FC has flown under the radar to become the league’s defining fixture.

It’s not quite Arsenal-Manchester United of the late 1990s and early 2000s, but there now exists an edge between the two clubs that previously wasn’t there. “When you play a team once and then twice in a major championship game and you each get a win, sure… that’s going to build a rivalry,” says Brad Humber, a TFC fan and member of the Red Patch Boys supporters’ group. “For a Western Conference team, they are our biggest rivals from that conference considering what has happened these past few years.”

This wasn’t the MLS Cup final most predicted. LAFC dominated the regular season from start to finish, picking up the Supporters’ Shield without much to slow them down. No MLS team had ever entered the play-offs so widely favored to go all the way. Atlanta United weren’t quite as strong, but as defending champions with the star power of players like Josef Martinez and Pity Martinez, it was difficult to see who would topple them in the East.

Few accounted for the momentum Seattle and Toronto FC would build over the play-offs, with the former unbeaten in their last seven seven home games and the latter unbeaten in their last 13 home and away. It was a similar story in 2016 and 2017, when momentum was the biggest factor in both teams’ run to the MLS Cup final.

What isn’t similar is the venue for this weekend’s clash. The last two Sounders-Toronto FC MLS Cup finals were played at BMO Field. This time, CenturyLink Field will host and there won’t be an empty seat in the house, with over 69,000 tickets sold almost as soon as they were made publicly available. It will be the most-attended soccer match ever in the state of Washington.

Of course, with this being Seattle’s first ever home MLS Cup final it’s likely that this demand would have existed no matter who the opposition, but the fact that it’s Toronto FC who will be in town has made the billing that little bit more attractive. With the Sounders winning in 2016 and TFC winning in 2017, this feels like something of a decider.

“Honestly, I didn’t even expect this to happen in the first place,” adds Humber, who along with his Red Patch Boys members, will be vastly outnumbered inside CenturyLink Field this Sunday. Nonetheless, around 3,000 TFC fans are expected at the game, with so many supporters making the journey out West that Humber will be forced to fly from Buffalo on Friday morning.

The rivalry between the Sounders and TFC hasn’t been sparked entirely from scratch. There already existed some form of sporting friction between the two cities owing to Seattle’s proximity to the Canadian border. “Whenever the Toronto BlueJays come to play the [Seattle] Mariners, the stadium is about 80% Toronto fans,” explains Cameron Collins, a member of the Gorilla FC Seattle Sounders supporters’ group. “There’s already a sort of Seattle-Toronto, Seattle-Canada rivalry. I think even though that’s a different sport there’s just that little bit of feeling there that adds to it. If it’s not a rivalry now, it will be after this.”

This is a match that has created memories in recent years, if not goals. “Hopefully we’ll actually score a goal in MLS Cup this time because even though we’ve won one we didn’t score in either game,” adds Collins, recalling the 0-0 draw, and ‘The Save’, that led to a penalty shootout win for the Sounders and the 2-0 defeat the year after.

There will be more of an onus on Seattle, being the home side, to take the game to TFC this time. This is a match that could be decided by which talismanic attacker – Raul Ruidiaz or Alejandro Pozuelo, with Jozy Altidore expected to stay sidelined – can get hottest in the Pacific Northwest rain that’s forecast for this weekend. These two teams are experienced and won’t be affected by the occasion, no matter how much of a spectacle 69,000 fans make it.

An MLS Cup final is always special regardless of who is involved, but this one arrives with a little extra sparkle, particularly from the Seattle Sounders point of view. This will be the first time that MLS’s original mega-franchise has hosted North American soccer’s showpiece event. “This time I get to go with my parents, my girlfriend and one of my best friends and we’re all going to sit together,” says Collins. “This time it’s more of a Seattle event.” That doesn’t meant Toronto FC can’t claim the night as their own, though.