These two powerhouses will meet in a two-leg Eastern Conference final, which begins Sunday night before another expected crowd of more than 70,000 at Mercedes-Benz Stadium. The second game will be Thursday night in Harrison, N.J.

"The approach is what it's been all along," said Red Bulls coach Chris Armas, who took over the job at midseason when Jesse Marsch left for a job in Europe . "The last 10 games of the season felt like playoff games. We felt like we couldn't slip up. That's what it feels like now."

The Red Bulls were a charter member of MLS when the league was founded in 1996 (known originally as the MetroStars), and they've been one of MLS' most successful clubs.

But they've never won an MLS Cup, coming closest to a title in 2008 when they lost to Columbus Crew in the final. Despite winning the Supporters' Shield for the third time in six years with 71 points, this team knows that anything less than a championship will be viewed as a failure.

"Yeah, it's been an amazing season," Armas said. "But we're 23 years going, and there's a cup out there we want really bad."

In just two seasons, United has quickly emerged as a league's marquee franchise, featuring two of the league's most dazzling players (Golden Boot winner Josef Martinez and Miquel Almiron, both MVP finalists) and boasting some of the best fan support of any club in the world. Atlanta broke its own attendance record this season, averaging more than 53,000 per game at its dazzling, retractable-roof home.

But even with all that success, there's something missing.

A trophy.