Yasuhiro Higuchi's side entered the final two games of the season four points clear at the top of the table but defeat to Albirex Niigata set up a tense final-day trip to the Todoroki Athletics Stadium.
And Marinos' hopes of a first league title since 1990 went up in smoke as Renatinho struck in the second half to seal a 1-0 scoreline, while Sanfrecce Hiroshima beat Kashima Antlers to climb to the summit.
Marinos have suffered an almighty hangover from that stunning collapse, and they head into the final game before the mid-season break just one place and one point above the relegation zone.
Having also finished bottom of their AFC Champions League group, Marinos will be desperate not to spend two months out of action due to the FIFA World Cup contemplating possible relegation.
Frontale will pose a tough test though, having picked up 13 points from the last 15 available to them on home soil.
Yoshito Okubo, who netted 26 times last season to top the J.League's scoring charts, already has eight to his name from 12 games and will hope to head to Brazil on a high after being named in Japan's squad for the tournament.
Sanfrecce themselves have not been able to take their momentum into the new season and sit fifth ahead of their trip to Vegalta Sendai this weekend.
Sitting pretty at the summit are Sagan Tosu, who can spend the World Cup at the top if they beat struggling Omiya Ardija - hoping for points to climb out of the bottom three.
If Ardija can spring a surprise, Urawa Reds will have designs on top spot as they host Cerezo Osaka.
Vissel Kobe and Kashiwa Reysol are the frontrunners for the third and final Champions League places ahead of clashes with Shimizu S-Pulse and Ventforet Kofu.
Elsewhere, Albirex host Nagoya Grampus, Gamba Osaka travel to Tokyo and bottom club Tokushima Vortis will hope to add to their paltry points return of four when they travel to Kashima Antlers - themselves still in sight of the top three.
