The blaze, burning mostly pine forest, sent smoke over the city of Patras, a port with 220,000 inhabitants, where regional authorities have declared an emergency.
Nine planes and one helicopter were involved in the firefighting effort at Argyra, nine miles east of Patras.
Apostolos Katsifaras, regional governor for western Greece, said evacuation orders had been issued for villages in the rugged fire stricken area, likely to involve several hundred residents.
“The conditions are very tough. We are using everything we have against the fire,” Mr Katsifaras said.
The state of emergency allows authorities to use additional resources, including Greece’s military.
Meanwhile, forest fires have led to 2,000 people being moved to safety from a town in the centre of Spain’s popular tourist island of Tenerife.
Residents were moved out of Vilaflor on the fringe of the Teide National Park on Tuesday night, said emergency services spokeswoman Carmina Lorenzo.
She said the park itself was not under threaten.
The fires are not close to the main coastal tourist resort areas.
Ms Lorenzo said firefighters were using 20 water-carrying planes and helicopters to try to put out the three-day-old blaze that has affected 5,000 acres.
Smaller fires are also raging on the nearby islands of La Palma and La Gomera.