D.C. United’s visit to Chicago on Saturday followed a familiar script: unfulfilled opportunity, an own goal (this one to United’s detriment), defensive errors and, ultimately and predictably, another defeat.

Ian Harkes placed the ball into his own net midway through the first half, and the rest transpired without disruption as United dropped a 3-0 result to the Fire before 15,643 at Toyota Park in Bridgeview, Ill.

Since showing some late-season life with three consecutive victories, last-place D.C. (8-17-4) has lost two in a row. After scoring in four straight matches, United suffered a 15th scoreless performance and remained last in MLS in scoring.

Harkes’s miscue came on a throw-in and set the match on an unfavorable path. Brandon Vincent scored 17 minutes into the second half and Nemanja Nikolic added a late penalty kick for the third-place Fire (14-9-6).

Coach Ben Olsen started two rookies from the D.C. area: Harkes (Fairfax, Gonzaga High School) and Chris Odoi-Atsem (Mitchellville, University of Maryland). Harkes played his first minutes since suffering an ankle injury July 29 and paired in central midfield for the first time with newcomer Russell Canouse, a partnership likely to take hold for good in 2018. Odoi-Atsem stepped in for right back Sean Franklin (yellow-card suspension).

Deshorn Brown replaced Patrick Mullins (scoreless this season) at striker and, with Zoltan Stieber in need of a break, Lloyd Sam manned the left wing.

Chicago, enjoying a renaissance season under second-year coach Veljko Paunovic, was without three starting midfielders: Bastian Schweinsteiger and Juninho are injured, and Michael de Leeuw served a yellow-card suspension.

United set an energetic tone and enjoyed quality build-up but did not seriously test goalkeeper Matt Lampson — a reprise of so many performances this year. Soon, the momentum swung.

Bill Hamid made three terrific saves on Chicago threats but, in the 23rd minute, had no chance of stopping an accidental shot by a teammate. On a long throw-in by Chicago’s Matt Polster, Harkes attempted to turn the ball out of danger and wide of the net. Instead, he directed the seven-yard effort into the near corner — the fifth time in eight matches that United has benefited or suffered from an own goal.

Hamid howled to the heavens. Chicago’s David Accam gave Harkes an appreciative tap on the back of the head. No D.C. players came to Harkes’s emotional rescue.

The goal clearly deflated United, which dropped its energy output and failed to regain the early promise. Under continued duress, Hamid swatted away threats to keep a manageable margin at intermission.

United had two good chances early in the second half, but Lampson made a leaping save on Luciano Acosta’s 18-yard strike and Brown failed to made solid contact inside the six-yard box.

Predictably, the missed chances cost United moments later. In the 63rd minute, Arturo Alvarez spotted Vincent surging forward from his left-back position and lifted a cross to the back side. Completely unmarked, Vincent stung a diving header from eight yards past the overwhelmed Hamid.

Mullins came on for Brown, but with United failing to score twice since mid-July, the outcome had been decided.

Kofi Opare’s poor clearance in the 89th minute forced Canouse to take down Luis Solignac in the box, resulting in Nikolic’s penalty kick for his 18th goal of the year — five fewer than United’s team total this year.

D.C. will host the San Jose Earthquakes next Saturday in the second-to-last MLS match ever at RFK Stadium.