Nelson Valdez overjoyed with Seattle winner, Peter Vermes furious

Video via MLS: Seattle 1-0 Sporting KC (1:57)

Seattle Sounders move on to face FC Dallas, thanks to Nelson Valdez's 88th-minute goal that sank Sporting KC. (1:57)

Nelson Valdez said it felt like a mountain had been lifted off of him after scoring a late winner to help the Seattle Sounders beat Sporting Kansas City 1-0 in the MLS Cup playoffs knockout-round.

However, SKC manager Peter Vermes was furious with the referees after the match and felt that Major League Soccer and the Professional Referees Organization owed the club an apology after getting several calls wrong in his team's loss.

Valdez, 32, had not scored a goal in Major League soccer in almost 365 days, his last coming against the Los Angeles Galaxy on Oct. 28, 2015.

But the Paraguayan international's timing to end his drought couldn't have been better, as he turned home a Joevin Jones' cross in the 88th minute to beat goalkeeper Tim Melia.

"It's felt like there's been a big mountain on top of me all season. Finally, with the goal, that mountain went away," Valdez said.

Valdez has had numerous chances during the season, but finally got one in the net, even though it appeared on replay he may have been offside on the winning goal. It was Seattle's only shot on goal in the match and the only goal scored by the Sounders in three matches against Sporting KC this season.

"I can't think of a better person to be rewarded for this moment," Seattle interim coach Brian Schmetzer said. "He has worked so hard all year long."

For Sporting KC it was another crushing postseason defeat that left coach Vermes disappointed in the outcome and upset at the officiating.

Vermes sternly said MLS owes his club an apology for the officiating, claiming Valdez was offside on his goal; Matt Besler was onside on a disallowed goal for Kansas City early in the second half and Seattle's Osvaldo Alonso should have been given a second yellow card and sent off midway through the second half.

"The statistics don't lie in this game. We dominated the game from beginning to end," Vermes said. "Like I said they scored their goal on an offside play. Changes the outcome of the game.

"I would tell you in my opinion and for our organization, PRO and Major League Soccer owe our club an absolute apology because that game was taken away from us."

This was the second consecutive year that Sporting KC's season ended in heartbreak in the knockout round of the MLS playoffs in the Pacific Northwest. Last season, the team lost in a memorable shootout loss to Portland that went through 11 rounds of penalty kicks before the Timbers finally won, the first step on their way to an MLS Cup title.

Kansas City had the majority of the possession and chances in the first half, pushing for an early lead and only being kept off the scoreboard thanks to Seattle goalkeeper Stefan Frei. Paulo Nagamura had two early chances that were punched away by Frei, the second save rebounded out for an attempt from Graham Zusi that beat Frei to the far post but ricocheted off the outside of the post and went out for a goal kick.

Kansas City appeared to take the lead in the 53rd minute when Besler's diving header off Benny Feilhaber's free kick beat Frei. But Besler was flagged for being offside and replays showed he was barely ahead of the Seattle defenders before the ball was played.

Feilhaber was the best player on the field most of the night, causing headaches for Seattle wherever he roamed. He dribbled through most of Seattle's defense in the 80th minute, only to be denied at the very end by another key save from Frei.

Seattle advances to the Western Conference semifinals against FC Dallas. Game one of the two-legged series is in Seattle on Sunday.