Dereham Town picked up three valuable points in Essex courtesy of Rhys Logan's first-half penalty.
Logan slotted home in the 33rd minute after Shaun Bammant was brought down by Witham defender Jimmy Wallace.
That was enough for the Magpies to claim a 1-0 win which saw them leapfrog Grays Athletic into 4th place in the Isthmian League North.
Grays could go back above their Norfolk rivals if they beat Barking on Tuesday night.
Dereham travelled south knowing only a victory would realistically keep them in the hunt for the play-offs.
Manager Adam Gusterson handed a debut to midfielder Joe Tarpey, on a month's loan from Cambridge United.
But it was struggling Witham who were the better side in the opening exchanges, and they squandered an early chance when Adam Vyse failed to convert Ayo Olukoga’s cross from the right.
With 16 minutes gone, Dereham had a penalty appeal waved away as Luke Johnson was seemingly clipped by a defender as he cut inside.
They had better luck, however, when the lively Bammant was put through on goal and fouled in the box by Wallace.
With the referee having pointed to the spot, Logan converted from 12 yards to put the visitors in front.
The remainder of the half saw Witham forward Ramiah Mills denied by a great block from Robbie Linford, and then a save by Antoni Bort in the Magpies goal.
Early in the second period, Dereham had yet another shout for a penalty ignored by the referee after debutant Tarpey was bundled over in the box.
As the home side tried to muster a reply, Rossi Jarvis and Matty Castellan were in commanding form at the heart of the Dereham defence, while Toby Hilliard continued to work hard up front.
The Magpies' best chances to add a second came late on. Logan fired off target following neat interplay after 85 minutes, before Tarpey drilled a shot over the top.
“It was a fantastic win," said Gusterson. "With so many bodies unavailable, it was a real gutsy effort to get the result.
"The pitch and conditions made it difficult but we adapted well and, having got the important first goal, defended excellently to limit Witham to hardly any attempts.
"It was a thoroughly professional performance."
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