Paul Ricard has been gripped by the European heatwave all weekend with high degradation on long runs in practice prompting Pirelli to suggest two-stop strategies are likely.
A further dynamic sees Ferrari running a higher downforce aerodynamic configuration than Red Bull.
"It is going to really be like a game of backgammon tomorrow, what moves you make and when, when you make your hit," Horner told Sky Sports F1.
"It is going to be a fascinating strategic race and I think it is going to play out over the whole race, not just the first sector."
Leclerc would have taken pole without slipstream
With Carlos Sainz taking a grid penalty for a power unit change, Ferrari used the Spaniard to create a tow for Leclerc during Q3 in order to negate Red Bull's straight-line advantage.
But conceding the tactic made little difference to the finishing order, Horner said: "I think we might have been a tenth and a half closer but I think over that one lap, Ferrari had that advantage on us today.
"We have got two very different concepts of attacking the race track this weekend.
"Ferrari running a bit more downforce, we are a bit more trimmed out so it is all about tomorrow."