Ben Olsen isn’t foolish enough to believe that, despite positive signs in preseason, D.C. United is a finished item, one that’s set to rejoin the MLS elite when the league campaign begins next week.

“This is still going to be a work in progress, even for the first part of the season,” the team’s head coach cautioned. “It’s a new group and we need to keep getting better and not get frustrated when things don’t pan out. There are going to be some ups and downs, and we’ve got to make sure we keep moving forward.”

On Wednesday, in the second of three matches at the Carolina Challenge Cup, United rested many of the opening-day probable starters in a 1-0 victory over the Chicago Fire at Blackbaud Stadium.

D.C., the worst team in MLS last year, is 7-0-1 in preseason, though only two matches have been against first-tier opponents. After winning the first two games here, United will conclude the preseason slate Saturday against Toronto FC and then begin preparing for the March 19 opener against the Columbus Crew at RFK Stadium.

Rookie forward Blake Brettschneider nodded in Santino Quaranta’s free kick in the 10th minute for his fourth preseason goal, placing him second behind newcomer Joseph Ngwenya (five). The University of South Carolina product, selected in the supplemental draft, was comfortable and confident throughout the half, ringing a 20-yard bid over the crossbar in the 26th.

“He was man of the match for me,” Olsen said. “He was pretty sharp. He is a handful and has got a lot of desire, and from the day he got here, he has played his way onto the team.”

Charlie Davies continued to work himself into form after arriving on loan from French club Sochaux, partnering with Brettschneider and playing the entire game.

Davies, who was severely injured in a 2009 car accident, didn’t receive much service from a makeshift midfield and was quiet in possession. The match’s slow pace didn’t play to his strengths, either. In the 90th minute, however, he displayed his acceleration before being ruled offside on what appeared to be a clean breakaway.

“This was a big step forward in him being connected all over the field,” Olsen said.

Chicago was short-handed for 53 minutes after Diego Chaves’s red card for a retaliatory tackle on Fred. Chaves, an Uruguayan forward, expressed his anger with referee Jose Carlos Rivero’s decision by kicking the inside of the team dugout three times in rapid succession.

The Fire had a claim for a red card — and perhaps a penalty kick — in added time when United defender Rodrigo Brasesco took down a free-running attacker from behind. However, Brasesco received a yellow card and the free kick came just beyond the penalty area.

United was good on the ball much of the match, controlling possession, working effective combinations and patiently initiating attacks out of the back.

Despite the influx of numerous players, including up to six starters, United players say they have forged a strong bond during preseason stops in Florida, California and now here.

“There is certainly a lot of building left to do, but in terms of what you want to get out of preseason from a relatively new team, it’s been fantastic,” said Dax McCarty, who started for FC Dallas last year and will serve as United’s midfield engine. “It’s so important to have team chemistry, not only on the field, but off the field. I’m a firm believer that you have to like the guy you are playing with to fight for him.”

Olsen and the front office have taken what they deem are worthy steps in renovating a roster that finished with a club-worst 6-20-4 record last year. Nonetheless, many uncertainties remain, including Davies, who hasn’t played in a top-tier league game in 17 months.

Goalkeepers coach Pat Onstad, 43, who came out of retirement to become a player-coach because of an injury crisis at the position, was largely unbothered Wednesday before yielding to rookie Joe Willis at the start of the second half.

United Note: Rookie defender Chris Korb (shoulder ailment) and defender Devon McTavish (concussion) were not in uniform. Probable starters McCarty, Dejan Jakovic, Jed Zayner, Branko Boskovic and Clyde Simms were given the night off ahead of Saturday’s finale.