The ŠKODA Motorsport driver was 7.4sec quicker than Bryan Bouffier through the Erbajolo-Pont d’Altiani.
It means Kopecky is 3.6s clear of Bouffier starting Saturday’s five stages.
Robert Kubica did not start the stage after stopping with a fuel-related problem at the end of the previous run.
"I have to say the weather is incredible over here,” said Kopecky. “There was a very long section on the dry then it started to be humid then changed damp, wet, dry, wet. It was changing a lot but we have changed the rear diff now and it's better."
Behind Bouffier, Stephane Sarrazin was fourth, Julien Maurin fifth and Francois Delecour sixth.
"The biggest problem was to hear my co-driver because it was very noisy,” said Delecour.
“We have broken two manifolds today so maybe it is a problem with the engine."
Subaru’s Andreas Aigner maintained his advantage in the ERC Production Car Cup by completing day one with a lead of 4.5s over Renault driver Germain Bonnefis.
Saturday’s first stage, Carbuccia-Tavera, starts at 10:48am local time.
- Bryan Bouffier
- Robert Kubica