Dwayne De Rosario and Brandon McDonald also scored to help United (12-8-4) become the first team to score more than two goals against the Fire this year. D.C. extended its home unbeaten streak to 12 games (9-0-3) - the second-longest in franchise history. United was 10-0-3 over a 13-game home stretch in 2007.
Gonzalo Segares and Daniel Paladini scored for fourth-place Chicago (12-8-5), which had its lead over United narrow to one point in the Eastern Conference standings. The top five teams in each conference qualify for the playoffs.
After Paladini's goal in the 45th minute goal tied the score at 1-all, Pajoy put United back ahead in first-half stoppage time, heading Andy Najar's curling cross into the lower corner from close range. It was Pajoy's first goal in two starts with United after being acquired from Philadelphia last week. The Colombian now has six goals in his first MLS season.
Chris Korb's cross found McDonald, who snapped an arching header over goalkeeper Sean Johnson's head to make it 3-1 in the 51st minute.
Segares cut it to 3-2 in the 75th on a header of Pavel Pardo's long free kick. But Tan, acquired in June in a trade with Vancouver, sealed United's win in the 89th minute when he slotted in De Rosario's cross.
Johnson made eight saves to keep the Fire in it after De Rosario put D.C. ahead in the 19th minute.
- Dwayne De Rosario
- Daniel Paladini