Wimbledon's hold on fourth place in London Division One South is hanging by a thread after Sidcup moved to within two points following their 17-10 win over the Dons.
The gap would have been narrower but for Wimbledon leaving south east London with a losing bonus point.
Wimbledon fielded a side with three changes and two positional changes from the previous match and Sidcup attacked straight from the kick-off.
A confident Wimbledon defence dealt with it and showed their challenge by running from behind their own line to the Sidcup 10m line before the three-quarters were tackled.
A penalty followed when Sidcup went to ground in the tackle and the resulting kick took Wimbledon to the Sidcup 22m line.
Flanker Jonno Reynolds made a big run down the left and, following the ball being secured by lock Rich Stewart, centre Kua Palakua went in for the try -Neil Hallett's conversion was successful for 7-0.
A further chance was offered a few minutes later but this time Hallett missed with the penalty.
Despite facing a stiff wind, Sidcup were starting to exert pressure with their powerful forward pack.
But held in their own half, Sidcup gave Wimbledon another penalty but again Hallett was unsuccessful.
Sidcup took their chance after the restart - a bundled drive to the Wimbledon right ended with Sidcup loose-head prop Tom Page being driven over by his forwards - the conversion was missed.
After the break, and with the wind in their favour, Sidcup’s second try came through captain Pete Grayson, and this time Richard Evans was successful with the conversion for a 12–7 to the hosts.
The Wimbledon scrum lost an important member after 50 minutes when tight-head Ben Davies went off with a shoulder injury and Joey Nanai switched sides, with Rich Stewart moving to loose-head and Danny Craven coming into the back row.
Sidcup soon had the measure of this makeshift arrangement but Wimbledon were generally faster to the breakdown and eventually they won a penalty which Hallett converted to reduce the arrears to 12–10.
Sidcup finished the scoring with an unconverted try from a line-out through flanker Mike Elliott and that was that.
There were positives for the team to take, a losing bonus point away from home, no yellow cards for once and the return after injury of some players who had not been seen recently.
Wimbledon’s 2nd XV also suffered defeat, going down 31–12 at home to Rosslyn Park Hatters.
Next weekend, Wimbledon 1st XV are at home to Old Colfeians, with a 2.30pm kick-off.