Even if it meant another 30 minutes against Portland, with a league match against the Timbers looming this weekend.
"No one ever put their head down so we kept battling," Cooper said after scoring on a header in the 110th minute to help the Sounders advance to the U.S. Open Cup semifinals with a 3-1 win over the Timers on Wednesday night.
Cooper's goal came in the second half of extra time after Darlington Nagbe's goal in the final moments of regulation forced an additional 30 minutes of play. Gonzalo Pineda sent a pass into the penalty area from 30 yards out and Cooper was able to flick it past Portland goalkeeper Donovan Ricketts.
"It was obviously disappointing because we thought we could win it in regulation we had to work extra," Seattle coach Sigi Schmid said. "But I told the guys, 'We have got to do it the hard way. We'll do it the hard way.' That's just the way it turned out tonight."
Seattle held on over the final minutes, helped by the Timbers playing a man down after Diego Chara was given a red card in the 99th minute for his tackle of Seattle's Marco Pappa.
Portland coach Caleb Porter was angry about the red card after the loss.
"That's why they won the game is the red card. At 1-1 we were the better team, we were the ones creating chances," Porter said. "(Seattle goalkeeper) Stefan Frei was tremendous. He saved then. That turned the game, that call. It was one of the worst officiating performances I've ever seen since I've been around the game."
Pappa added an insurance goal in the 116th minute beating Ricketts from the top of the penalty area with a right-footed shot. Seattle, which won three straight U.S. Open Cup titles between 2009-11, advanced to host the Chicago Fire in the U.S. Open Cup semifinals on Aug. 13.
It was the first of two games this week between the heated Cascadia rivals. The Sounders will host the Timbers in MLS play Sunday night.
"It's over, it's history, it's soccer," Porter said. "We were on the wrong end of a bad call today, and it was definitely the difference in the game."
Osvaldo Alonso scored on a leaping right-footed shot in the 69th minute to give Seattle a lead that held up until Nagbe's goal in the third minute of second half stoppage time.
Nagbe beat Lamar Neagle to a loose ball along the sideline, took a return pass from Steve Zakuani and beat Frei to the far post.
The first 90 minutes lacked the fireworks of the wild 4-4 draw the Sounders and Timbers played to in Portland in April, the first of three MLS matches. But both sides fielded a nearly first-team squad.
That included the return of Seattle's DeAndre Yedlin following his standout performance for the United States at the World Cup. Yedlin played all of regulation on his 21st birthday, but was subbed off for extra time. Seattle is expected to have U.S. captain Clint Dempsey back in time for Sunday night's match where more than 60,000 are expected at CenturyLink Field.
"He said he felt fine," Schmid said of Yedlin. "We felt he is a guy that could play tonight and turn it around again for Sunday."