Seattle Sounders kept their MLS Cup title defence alive after overcoming Vancouver Whitecaps 2-0 on aggregate in the Western Conference semi-finals.

United States international Clint Dempsey was the hero in Thursday's second leg, with his second-half brace sending the defending champions through to the Conference finals.

The play-off tie was delicately poised following last week's dour scoreless draw at BC Place Stadium in Vancouver.

But the return of veteran forward Dempsey from suspension proved pivotal as the Sounders emerged triumphant in wet conditions at CenturyLink Field in Seattle.

Dempsey – who almost opened the scoring with an outstretched volley in the first half – broke the deadlock in the 56th minute when he finished powerfully past Whitecaps goalkeeper Stefan Marinovic from the top of the penalty area.

And the 34-year-old sealed Seattle's progression with two minutes remaining after scoring from virtually on the line following a Victor Rodriguez cross.

The Sounders will face either Houston Dynamo or Portland Timbers for a spot in the MLS Cup final.