Penalty shootout defeat at the Jonny-Rocks Stadium saw Cheltenham exit the Gloucestershire Senior Cup at the quarter-final stage.
Having been behind for much of the night against a determined Evo-Stik Southern League Division One South Slimbridge, Billy Waters brace had looked to rescue a win for the Robins.
However Steve Jackson headed in during stoppage time to send the game to penalties, with the away side eventually coming out on top.
Michael Duff took the opportunity to offer playing time to first team group Manny Duku, Waters, Cameron Pring, Matt Bower, Alex Addai and Rhys Lovett.
Earlier though, Duku had headed a Bower cross wide, while Waters had fired off target after a good cut back from Pring.
Will Dawes did well to find space in the box to produce a shot that went out from a corner after linking up with Waters, but it was the visitors who went ahead with their only chance of the half shortly after.
Jamie Martin’s persistence saw him rewarded as he made life tricky for the Robins back line and was able to fire in to break the deadlock.
The Robins continued to create chances despite going behind. Waters saw a shot saved by Lewis Adams after Pring had teed him up following a driving run down the left, and Addai drove a powerful strike wide just before the half hour mark.
Archie Brennan’s steered cross found Duku a short while later but he could only nod off target.
After the break substitute Aaron Basford replaced Duku, and immediately set about firing in an effort on goal that Adams caught comfortably.
Adams made an even better save just past the hour mark as he turned Tyreece Briscoe’s smash from distance past the post, while he smothered Basford’s close-range effort from a difficult angle well minutes later.
He would eventually be beaten as the Robins persistence paid off. A quality through ball from Briscoe set Waters clear, and he swept home to level.
It would be Waters who would turn the contest around and give his side the lead.
A shortly taken corner saw him deliver a low ball into the box, and after Slimbridge struggled to clear, he hammered in from close range.
It looked like it might be the winning goal right up until the first minute of stoppage time when Jackson headed in Martin’s superb cross from the right to amazingly draw the away side level.
The game went straight to penalty kicks and the visitors were ruthless from the spot. Will Hawes and Christien Turner were among those to emphatically fire in from 12-yards, and Danny King did the honours of sealing the win as he stroked in to confirm the Swans place in the semi-finals.
ROBINS: Lovett, Horton, Handley, Bower, Briscoe, Dawes, Brennan, Pring, Waters, Addai (Price 66), Duku (Basford 46)
Subs not used: Lapworth, Scott, Duncan
SLIMBRIDGE: Adams, King, Noakes, Richards (D Turner 50), Jackson, C Turner, M Turner, Smith, Martin, Lomotey (Hawes 73) , Mendes (Dunbar 86)
Subs not used: Beadle