MIDDLESEX County League side Southall suffered their fifth defeat in six matches when they lost 2-1 at Copland - the winner coming in injury time.
The home side had slightly the better of the first half and took the lead shortly before half-time, but Southall created several chances in the second half.
And their determination was rewarded with 15 minutes remaining when Troy Bantleman equalised with an accurate free kick from 20 yards.
That looked like being enough for a deserved draw, but in the deepening gloom, Copland snatched victory.
Player manager Ryan DeCosta was named the supporters' man of the match.
Tomorrow (Saturday, 2pm) Southall, who are 12th in the 17-team Premier Division, return to Argyle Road for the visit of Bethnal Green United A.