The winner of the Union-Dallas game will host the tournament's final on Sept. 16. The Chicago Fire will visit the Seattle Sounders in the other semifinal on Wednesday.
The semifinal is "as big a game as we have," defender Sheanon Williams said. "If we get to the finals, it is probably the biggest game in the club's history."
The Union's interim manager, Jim Curtin, has to consider resting some players who played Saturday, and there is another factor to consider: On Friday, the Union will visit the Houston Dynamo in an MLS game.
"The crazier thing is that we have to turn around and play again on Friday in Houston, which is tough," Curtin said. "We're not going to make excuses. We have a deep squad now - if you look at the bench we have hundreds and hundreds of games of experience, so that's a positive to come in and close out games."
One player who likely won't make an appearance is two-time World Cup goalie Rais Mbolhi, an Algerian star who signed with the team on July 30 but has not played.
Either Zac MacMath or Andre Blake will likely be in goal. Blake, who has yet to play in MLS action, was the keeper in the Union's first two Open Cup wins. MacMath was in goal for their 2-0 quarterfinal victory over the New England Revolution.
Union at FC Dallas
Semifinals of the U.S. Open Cup, Tuesday at 9 p.m.,
Toyota Stadium, Frisco, Texas
MLS records: Union 6-8-9; FC Dallas 10-7-6
The road to the semifinals:
The Union beat the Harrisburg City Islanders (USL Pro), 3-1 in OT; the New York Cosmos (NASL), 2-1 in OT; and the New England Revolution (MLS), 2-0.
FC Dallas beat the San Antonio Scorpions (NASL), 2-0; the Houston Dynamo (MLS), 3-2 in OT; and the Carolina Rail Hawks (NASL), 5-2.
Outlook: The Union's Sebastien Le Toux has scored seven goals in his last eight MLS games. Le Toux hurt his ankle in Saturday's 2-1 win over the Montreal Impact. He said he hoped it was only a deep bruise, but his availability is unknown. . . . Union midfielder Cristian Maidana, who has missed the last three MLS games with a hamstring injury, could return Tuesday, interim coach Jim Curtin said. . . . The Union will stay in Texas and play the Houston Dynamo in an MLS game on Friday. . . . FC Dallas scored a 2-1 MLS win over the visiting Union on July 4. They return to MLS action Saturday at San Jose. . . . Midfielder Michel leads Dallas with seven MLS goals. Forward Blas Perez has six goals and a team-high five assists. - Marc Narducci