Hannspree Stiggy Honda's Josh Brookes had overturned Broc Parkes' pole position to take an early lead over the Yamaha rider, while Broc's team-mate Foret initially lost ground from third on the grid.
But the former world champion soon rebounded, passing Brookes for second position on lap five and then breezing past Parkes to take the lead at the halfway mark of the 16 laps.
Foret then rode to a confident 1.199secs victory over Brookes, with Parkes fading to third, almost seven seconds behind the race winning R6.
Andrew Pitt spent much of the race at the head of a furious five-man fight for fourth, a position the Australian just secured in front of Brookes' team-mate Robbin Harms, Craig Jones - riding with a fractured left hand - Massimo Roccoli and Matthieu Lagrive.
Pitt's Hannspree Ten Kate Honda team-mate Jonathan Rea retired with a technical problem early in the race, while fellow Britons Chris Walker and Graham Gowland were among several riders to fall at the first turn.
World championship leader Joan Lascorz, who had qualified just 16th, salvaged ninth position but is now just six points clear of Foret.
WSS race results - top 10:
1. Foret (Yamaha)
2. Brookes (Honda) +1.199secs
3. Parkes (Yamaha) +6.736secs
4. Pitt (Honda) +11.398secs
5. Harms (Honda) +11.477secs
6. Jones (Honda) +11.716secs
7. Roccoli (Yamaha) +11.757secs
8. Lagrive (Honda) +12.186secs
9. Lascorz (Honda) +14.847secs
10. Rodriguez (Yamaha) +15.175secs
20. Coghlan (Honda) +46.327secs
Walker (Kawasaki) DNF
Gowland (Honda) DNF
Rea (Honda) DNF