Two first half goals from Jacob Fenton were enough to secure three points for Anstey Nomads on a bitterly cold evening at Cropston Road.
Town boss Carl Adams opted to bolster his back-line by fielding three central defenders in a starting line-up that showed two changes from the weekend's 6-0 drubbing at home to Quorn with Liam Francis and Caine Elliott back in place of Harrison Nee and the injured Ryan Sahota.
The hosts made a strong start to the game, earning themselves a series of corners in the process - with Fenton and Brad Wilson having shots blocked, and Leo Brown striking one well wide from these.
Rugby were also making an impression of their own though, with Trey Charles chipping over a decent cross which was taken high by Nomads keeper Conrad Logan, and a low cross from Keiron Berry splitting Silvano Obeng and Charles in the box.
Town's best chance of the half soon followed, and it came via a swung-in free-kick from Charles to which Francis could only get the slightest of touches on in front of an inviting goal.
A further delivery from the right-hand side by Charles failed to land kindly for both Francis and Obeng, with Berry subsequently seeing his shot from the loose ball crowded away for a corner.
Oscar Tonge's wayward effort was followed a similarly misdirected one by Leo Brown, before Anstey took the lead on 24 minutes when Brown played in Zayn Hakeem down the left, and he dinked up a cross to the back post for Fenton to apply a downward header past Hathaway.
Brown headed a good chance over shortly after that when Tong picked him out with a delivery from wide, with Caine Elliott almost finding a way through the home defence at the other end - before stumbling at the crucial moment.
The hosts grabbed a critical second goal just before the interval though, when Brown and Hakeem again combined well to give Fenton his second of the frosty evening, when he beat Damien Osoimoejiokhian in a tackle in the six-yard box to help the ball into an unguarded net after Hathaway could only parry Hakeem's low, powerful shot.
Valley started the new period brightly, with Berry having a strike blocked in its opening moments.
The visitors then had a great opportunity to reduce the deficit when Elliott's lofted pass gave Charles a run on goal, but with Logan out to cut the angle down, Town's top scorer could only curl his effort wide of the target.
From that point on, it was the home side that looked the more likely to score again, with their midfielder Tristan Thompson-Matthews going close on a number of occasions.
He dragged a decent chance wide after neat interchange play from the Nomads, and then he clipped the top of the bar with a good effort from the edge of the box.
Prior to those close-escapes, Osoimoejiokhian had denied Fenton his treble with a well-timed aerial challenge, and after them off-target efforts from Kyle Tomlin and Brown also threatened to rack up a third for Anstey.
Charles did blaze one high and wide after a quick free-kick from Clements, and Dan Summerfield's cross from the right almost fell for Berry, before Thompson-Matthews had two further efforts - a volley which Hathaway did well to keep out and a wild slash over the bar.
Francis and Hathaway combined to snuff out a close-range opportunity for sub Rhys Dolan in the final minute of the game, before the whistle signalled a tenth defeat of the league campaign for Valley.
