The hosts started brightly with an early strike by Brad Abbott going just over over bar.
Guiseley had a chance of their own when Brad Nicholson took aim from just outside the area, but his shot drifted wide.
There was another chance for the Lions when Josh Stones was played in, but good goalkeeping meant the scores stayed goalless.
Spennymoor took the lead when Jube Oyibo beat Hamza Bencherif for pace and put in a low cross. Glen Taylor touched it on to Rob Ramshaw who had a simple tap-in from less than two yards out.
The hosts could have doubled their lead at the end of the first half but good defending ensured the game was still there for Guiseley.
The start of the second half was even, until substitute Adam Haw intercepted a pass and was able to find Thewlis and he took a touch and was able to get the ball over the keeper and into the net.
Momentum was with the visitors, but as is normally the case for teams near the bottom of the table, they had four chances blocked in a row and a late sucker punch arrived soon after.
Spennymoor’s Taylor kept his composure and produced a smart back heel to Ryan Hall who was able to run through and fire home.
The result means Guiseley are now just five points off bottom spot, having played a game more than rock-bottom AFC Telford United.
Speaking after the game, Guiseley joint boss Marcus Bignot said: “We got the goal to level things and had a spell after and just couldn’t take advantage.”
The Lions are next in action in the FA Trophy on Saturday at Stourbridge.
Farsley were thrashed 6-0 at home to Darlington following a crazy first half in which the Celts rattled the frame of the goal three times, only for Jack Lambert to hit a hat-trick at the other end of the pitch.
Lewis Turner hit an upright for the hosts before Lambert’s curling effort opened the scoring at the first half’s midway point.
He then added a second on the half-hour mark and, after Luke Parkin struck a post for Neil Ross’ men with a free-kick, Lambert completed his treble on 42 minutes.
In first-half stoppage time, the unfortunate Parkin also headed against an upright as Farsley failed to find the net for a fourth straight game.
Second-half efforts by Jarrett Rivers, Tyrone O’Neill and Kevin Dos Santos went on to compound Farsley’s misery and only a fingertip save by Kyle Trenerry prevented a seventh goal as he pushed Will Hatfield’s shot onto the crossbar.
The Celts are next in action at Kidderminster Harriers on Tuesday – the conquerors of Championship side Reading in the FA Cup third round on Saturday.
Elsewhere, Brad Dockerty grabbed the only goal of the game as Bradford Park Avenue enjoyed their first league win since November 6 against Chester on Saturday.
The contest’s decisive moment came in the ninth minute courtesy of Dockerty’s composed finish following an excellent pass by Akeem Hinds, who was making his second debut for Avenue.
Mark Bower’s side then defended resolutely to keep a first clean sheet at home since November 2020, ending their 15-fixture sequence without a shut-out at the Horsfall Stadium.
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