First-half strikes from Harry Jackson and Zak Willett were followed by Tommy Tierney's effort after the restart as the Deans made the most of their chance to briefly open up some slight breathing space in what is increasingly looking like one of the tightest races for the title in a number of years.
Brockenhurst's shock defeat at Christchurch on bank holiday Monday saw them miss the chance to leapfrog Horndean and temporarily climb top, as they slipped down to fourth following their loss with Hamworthy and Baffins Milton Rovers moving into second and third spot respectively after their victories – with the three clubs currently below Horndean in the table all locked on 51 points.
But the Deans, out of sorts somewhat in the run up to Christmas with a run of just two wins in five league matches, did at least make the ideal start to 2022 to open up a three-point gap at the summit.
This meeting between the PO postcode rivals had been put into doubt, but was given the green light to go ahead after an afternoon pitch inspection at Five Heads Park.
Both Horndean and US Portsmouth were returning to action after 19-day lay-offs over the festive period, although Michael Birmingham's side made a quick start and were ahead after 14 minutes. Former US Portsmouth forward Jackson was the man to get it, chipping over visiting goalkeeper Dylan Kramer from the edge of the area.
Willett was then on hand to bag goal number 26 in 27 Horndean appearances this season, doubling the hosts' advantage on 43 minutes.
The Deans were threatening to run away with things when Tierney hammered home after being teed up by Jackson after the restart.
But US Portsmouth, on the back of a fine 4-1 win over Cowes Sports in their previous outing on December 18, pulled a goal back when Conner Grant poked home at the back post on 61 minutes. Yet there was to be no rousing fightback from the visitors as Horndean sealed a 17th league win in 24 matches this season to go clear at the top.
Horndean: Scott; Taw, Field, Dempsey, Emeney, Hookey, Anderson, Tierney, Jackson, Willett, Duffin. Subs: Dean (Emeney, 50), Kimber (Tierney, 69), Harris (Duffin, 89), Brownlie, Martin.
US Portsmouth: Kramer; Willcox, Vucaj, Grant, Hadi, Sibley, Quirke, Smart, Dinsmore, Woods, Mason. Subs: Paige (Sibley, 84), Matonga, Magee, Osborne.