There were no fewer than five changes to Simon Rowland's starting XI, with Adam Davidson, Ben Dartnall, Lucas Mann and Aaron Capon returning while there was a first start of the season for Scott Dartnall.
An even opening period saw both 'keepers have their palms stung, but neither was forced to make a save of note.
Capon had a glorious chance to give United the lead as half time approached, but he was unable to generate enough power to test Ender.
With less than three minutes played in the second half, it was Chichester who took the lead.
A free kick from the touchline fell loose at the back post and Morgan Forry was the first to react as he looped a header over a stranded Tanner.
Wade Eason came inches away from an equaliser as his glancing header went agonisingly wide following a Wes Tate free kick.
Lewis Edwards hit the bar for the visitors from a 25 yard free kick as Chichester got men behind the ball and stifled any attempted attack from the United side.
A change of formation saw United go with a more attacking setup, but with men pushing forward, Chichester got their second of the evening as a counter attack was finished off by Tom Groom flashing an effort across Tanner.
Pete Featherstone saw a curling 20yd effort tipped over in the final minute as it proved not to be United's night as Chichester progressed to face either Littlehampton Town or Shoreham in the Quarter Final.