Byrne went fastest in the final four minutes in drying conditions.
If Byrne is crowned champion he will become only the third man to have his name carved on the BSB trophy three times, joining Niall Mackenzie and Ryuichi Kiyonari in the exclusive club.
The 35-year-old, champion in 2003 and 2008, leads Josh Brookes by nine points in the standings, with Alex Lowes a further 15 behind with 75 up for grabs from the three races at the Kent circuit.
Despite coming into the season-ending Showdown with a dislocated shoulder and third in the standings, a victory in Assen and three second-place finishes has seen the Rapid Solicitors Kawasaki ace rise to the top.
And, after winning the final two races at Brands last year, the Kent local knows a repeat performance would all but guarantee him a perfect homecoming.
Free practice one:
Shane Byrne (Rapid Solicitors Kawasaki)
Karl Harris (Quattro Plant Kawasaki) +0.934s
Alex Lowes (WFR Honda) +0.936s
James Westmoreland (WFR Honda) +1.339s
Chris Walker (Pr1mo Bournemouth Kawasaki) +1.438s
Josh Brookes (Tyco Suzuki) +1.490s
Tommy Hill (Swan Yamaha) +1.680s
Tommy Bridewell (Supersonic BMW) +1.849s
Noriyuki Haga (Swan Yamaha) +2.565s
Peter Hickman (Buildbase BMW) +2.691s
- Josh Brookes