The side are enjoying an impressive first season playing a year above themselves in the Lincs Intermediate U18s League, and took the hosts to penalties before succumbing to the lottery of the shoot-out.
With the tie re-arranged at short notice, Sleaford were struggling for numbers, but this didn’t appear to hinder them as they went ahead on 17 minutes, when Michael Fleming lofted a wonderful long-range strike over the keeper.
Both teams had to work hard as the pitch turned into a quagmire, but it was the visitors who posed the greater threat with Owen Park, in particular, showing wonderful technique, balance and stamina to attack the home defence.
Town doubled their lead when good approach play from Kyle Blair on the right created chaos in the penalty area and Fleming reacted quickest to thump the in the second.
The Green Army extended their lead further on the stroke of half-time when Fleming completed his hat-trick with a precise free-kick around the wall.
Carrying a number of walking wounded into the second half, Sleaford’s cause wasn’t helped when Cleethorpes dispatched a penalty within two minutes of the re-start.
Scott Jenkins and company in Sleaford’s defence pulled together to repel the home attacks, but a second goal arrived with 35 minutes still to play.
With pressure mounting, Sleaford broke out on the counter and went close to increasing their lead through long-range shots from Lewis Russell, Park and Blair.
Cleethorpes’ pressure finally paid off as they stunned the visitors with an equaliser in the dying minutes to send the tie into extra-time.
Sleaford were a player down through injury, but still matched the Cleethorpes XI as the pitch conditions sapped their remaining energy reserves.
With neither team able to score the all-important next goal, the game went to penalties.
Town keeper Scott Jenkins pulled off two moments of magic, but the hosts took the shoot-out 2-0.
* Metheringham host Grimsby Borough in the latest George Addlesee Trophy quarter-final on Sunday, kick-off 1.30pm.