(The Sports Xchange) - Matteo Mancosu scored twice in his first career MLS playoff game to help Montreal Impact earn a 4-2 victory over D.C. United in an Eastern Conference knockout-round match on Thursday.

In the Western Conference, Nelson Valdez scored with a diving header in the 88th minute to lead the Seattle Sounders to a 1-0 win over Sporting Kansas City.

The Sounders will play top-seeded FC Dallas in the semi-finals while Impact will face New York Red Bulls.

In Washington D.C., Montreal's Laurent Ciman scored his first goal since the 2015 regular season with less five minutes on the clock before Mancosu scored either side of half-time.

Ignacio Piatti made it 4-0 with seven minutes left before Lamar Neagle and Taylor Kemp scored in the dying moments for fourth-seeded D.C.

In the other playoff game, Valdez was mobbed by his team mates after scoring the late winner at CenturyLink Field.

Seattle were ninth in the 10-team Western Conference after a loss to Kansas City on July 24 and two days later long-time coach Sigi Schmid was fired.

Under interim coach Brian Schmetzer, Seattle climbed the table to fourth, the turnaround all the more surprising given Clint Dempsey has been out since late August due to an irregular heartbeat and will not return to action in the postseason.

(Compiled by Peter Rutherford)