D.C. United interrupts its Major League Soccer schedule tonight to play the A-League's Charleston (S.C.) Battery in the U.S. Open Cup, an 85-year-old tournament whose purpose is considered odd by American sports standards but commonplace in the soccer world.

Imagine the NBA champion San Antonio Spurs in the same tournament bracket as an upstart amateur team from Brooklyn. That essentially describes the U.S. Open Cup, which features amateur and professional teams from all over the country and is modeled after prestigious soccer tournaments overseas such as England's 127-year-old FA Cup.

Entrants range from MLS's best teams to obscure amateur clubs such as the Mid-Michigan Bucks, who pulled off two upsets before losing to MLS's Tampa Bay Mutiny.

Eight MLS clubs were given byes until the round of 16. Five have been victorious and two, Chicago and New York/New Jersey, lost to teams from the A-League, which is the U.S. equivalent of a second division.

United -- the tournament champion in 1996 and the runner-up in 1997 -- takes its turn in the round of 16 tonight at Blackbaud Stadium, a new, 5,200-seat facility located on Daniel Island in Charleston. If United wins, it would play a quarterfinal on the road in two weeks against the A-League's Staten Island Vipers, who upset the MetroStars last month.

The semifinals will be played in early September and the final is set for Sept. 14 in Columbus, Ohio.

Tonight, United is favored to advance despite the absence of four starters who are with the U.S. national team at the Confederations Cup in Mexico and captain Marco Etcheverry, who didn't make the trip because of a sore heel.

Also, midfielder John Maessner cannot play because he was acquired after the July 1 roster deadline for the tournament. Rookie midfielder-forward Chris Albright joined United at the same time Maessner did, but the club is hoping he will be declared eligible today.

"Sure, you expect to win, but history has shown there are a lot of upsets in situations like this," United Coach Thomas Rongen said. "Charleston will be tremendously motivated to beat us, but we're motivated too. At the end of the road, there is a trophy to win and money to earn" -- about $100,000.

Since the league's launch in 1996, MLS teams have won each championship (United in 1996, Dallas in 1997 and Chicago last year).

Before that, most of the winners were from amateur and semipro ethnic leagues or were sponsored by companies -- teams such as the Ukrainian Nationals of Philadelphia, Greek-American AC of San Francisco and Bethlehem Steel FC. (In the 1970s and '80s, North American Soccer League teams didn't enter.)

Charleston, which lost two league road games last weekend, is 11-10 and tied with the Hershey Wildcats for second place in the A-League's Atlantic Division. United (13-7 in MLS) hasn't played since a 2-0 victory at New England 10 days ago. Its next league game is Sunday against Tampa Bay at RFK Stadium.

UNITED PREVIEW

Data: vs. Charleston (S.C.) Battery.

What: U.S. Open Cup, round of 16.

Where: Blackbaud Stadium.

When: 7:30 p.m.

D.C. Probable Starters: GK -- Tom Presthus; D -- Diego Sonora, Eddie Pope, Carey Talley; MF -- Geoff Aunger, Antonio Otero, Jason Moore, A.J. Wood, Mike Slivinski; F -- Jaime Moreno, Roy Lassiter.

Battery Top Players: GK Dusty Hudock, F Mike Burke, MF Paul Conway, D Dean Sewell, MF Micky Spooner.