Robert Kubica ended up classified 44th in his first circuit race since the 2010 Abu Dhabi Grand Prix after an opening-lap incident.
The Formula 1 race winner contested the Mugello 12 Hours, a round of Creventic's International Endurance Series, in a GT3 Mercedes entered by his former World Rally Championship rival Martin Prokop and Quirin Muller's MP Sports team.
Kubica held provisional pole for much of qualifying, before being demoted to third by Christian Engelhart in the GRT Grasser Lamborghini and Scottish driver Rory Butcher's MJC-Furlonger Ferrari.
Contact on the first lap of Friday's opening four-hour segment of the split race ultimately led to suspension damage that cost the MP crew 35 laps early on.
They rejoined and got back up to 34th in the 64-car field as Kubica showed competitive pace, before a late gearbox problem brought the Mercedes back into the pits.
The race was won by V8 Racing with a Renault RS01 one-make racer.
British GT team Optimum Motorsport had held a dominant lead until being delayed by a late gearbox problem.
Optimum rejoined but its Audi stopped on track with a further issue and was collected violently by a Porsche under yellows. Driver Ryan Ratcliffe escaped injury.
Kubica, who billed the GT outing as a one-off for now, had only competed in rallying since his F1 career ended due to the horrific arm injuries he sustained in a crash on an Italian rally in February 2011.
There has been speculation he will make a full-time return to circuit racing as his WRC programme came to an end after this season's Monte Carlo Rally.
The Pole's rallying switch had begun with great promise - as he claimed the 2013 WRC2 title, won outright in the European Rally Championship and numerous Italian domestic events and led the 2014 Monte Carlo Rally on only his second top-level start in a World Rally Car.
But he had difficulties with the level of mobility in his damaged arm on tighter courses and a string of costly mistakes and technical problems when running strongly on rallies meant he did not finish on the overall podium in his three-year WRC stint.