Elfyn Evans's Rally Argentina lead was hacked down to just 11.5 seconds on Saturday afternoon as problems with his DMACK Ford allowed rivals to close in dramatically.
Reigning British Rally champion Evans's advantage peaked at just over a minute earlier on Saturday, before two damaged tyres meant the gap was reduced to 44s over Thierry Neuville by the midday service.
Evans put a second back on his lead on the first afternoon stage, but reached the finish concerned with a potential problem at the rear of his car.
That turned out to be the loss of the car's diffuser, and he lost 15.1s to second and third placed Neuville and Ott Tanak as they shared the fastest time on the long Los Gigantes stage.
The problems escalated on the day's finale, Boca del Arroyo, as after a competitive first split Evans began dropping more time with a problem at the rear of the car that he could not explain, and that led to a spin at one point.
Evans lost 18.9s to Neuville as the Hyundai driver won the stage, leaving them only 11.5s apart with three stages left on Sunday.
Tanak could not match Neuville's searing pace on that stage and ended the day 15.3s off second place and 26.8s from leader Evans, but he did outpace M-Sport team-mate Sebastien Ogier throughout the afternoon. They are now 23.1s apart.
Mads Ostberg had started the afternoon ahead of both M-Sport drivers in his privately-entered Ford, but lost out to Tanak when he was over-cautious through the Tanti watersplash that had ripped his diffuser off in the morning.
Worse followed on Los Gigantes when Ostberg clouted a rock near the stage end and had to retire from leg two with broken suspension.
Jari-Matti Latvala completes the top five after steadily losing touch with the Fords ahead through the loop.
Hayden Paddon is a distant sixth, further delayed in the afternoon by a penalty for starting SS14 late when he tried to fix a broken rollbar. His efforts were in vain, but he still managed third-fastest times on both the following stages.
Juho Hanninen had nearly caught Lorenzo Bertelli when his Toyota ran into further mechnical trouble and was left limping through the final two stages.
But he still moved up to seventh at the Italian's expense as Bertelli then had to stop to change a puncture on the last stage.
Dani Sordo's recovery drive following his Friday morning damage has now brought him up to ninth ahead of WRC2 leader Pontus Tidemand.
Kris Meeke had returned under Rally2 after crashing on day one, but rolled again early on Los Gigantes and retired for the second time in as many days.
LEADING POSITIONS AFTER SS14: