Contador finished 35th in the 19th stage, a 52-km timed effort won by Swiss Fabian Cancellara, and holds a 39-second advantage over Luxembourg's Andy Schleck, according to provisional results.
The final stage on Sunday is traditionally a day-long parade to the Champs Elysees in Paris.
Schleck, who started the stage eight seconds behind the 2007 and 2009 champion, began briskly and Contador passed the first time check two seconds slower than his rival.
Contador dug in and was seven seconds faster at the 36.2-km mark and extended his advantage to 31 seconds at the finish line.
Russian Denis Menchov, who started the day in fourth place overall, is set to finish third after leapfrogging Spain's Samuel Sanchez.
Cancellara won the stage in one hour and 56 seconds, beating Tony Martin by 17 seconds, with fellow German Bert Grabsch coming home third 1:48 behind.
