Le Toux converted a penalty in the 25th minute after Tony Beltran had upended Roger Levesque in the penalty area, and two minutes later took a pass from Levesque and played a ball into the middle for Sanny Nyassi to belt into the net. RSL midfielder Will Johnson scored in the 32nd minute to cut the deficit in half at 2-1 and the visitor managed to maintain that margin until halftime, only to be double-punched again shortly after halftime.
Levesque and Le Toux combined for a second time seven minutes into the second half to set up Stephen King, who blasted a shot from 16 yards out past RSL keeper Chris Seitz. Again, two minutes later, the Sounders scored when Levesque crossed a ball for LeToux to finish from just outside the goal area.
"It was nice to come back here," said Le Toux to the Seattle Times of a rowdy atmosphere generated by a full house of 3,890 fans. "It was good for me to score and to find the confidence."
The Sounders advance to meet Colorado, which topped Los Angeles in a play-in match three weeks ago, on a date to be determined. Success in this tournament hasn't been forged by the move up to MLS status.
As a USL-1 club, Seattle reached the semifinals in both the 2007 and 2008 competitions, with Le Toux netting five goals each year. Keeper Chris Eylander, the starter Tuesday against RSL, posted a 435-minute USOC scoreless streak last year and was named Player of the Tournament. Coach Sigi Schmid won the Open Cup with the Galaxy in 2001 and 2005.
New York plays San Jose Wednesday at Buck Shaw Stadium in another play-in match.
U.S. Open Play-In
April 28 in Tukwila, Wash.
Seattle 4 Real Salt Lake 1
Goals: Le Toux (pen.) 24, Nyassi (Le Toux, Levesque) 26, King (Le Toux, Levesque) 53, Le Toux (Levesque) 55; Johnson 32.
Seattle -- Eylander, Riley, Graham, Wahl, Scott, Nyassi (Brown, 72), King, Ianni, Zakuani (Smith 65), Levesque, Le Toux.
Real Salt Lake -- Seitz, Beltran, Horst, Olave (Borchers, 46), Russell, Grabavoy, Beckerman (Alexandre, 66), Williams, Johnson (Cox, 71), Findley, Escalada (Movsisyan 57).
Yellow cards: SEA -- Ianni 19, Nyassi 71; Olave 41.
Referee: Landis Wiley