The fire is only 10 per cent contained, down from 15 per cent on Saturday, after it suddenly spread seven miles to ravage another 56,000 acres yesterday.
Santa Ana winds and the rugged mountainous terrain have hindered firefighters as they battle the blaze, which has destroyed 790 houses, outbuildings and other structures and left 90,000 homes and businesses without power.
Fire Captain Steve Concialdi, spokesman for the Thomas Fire: “A lot of these guys (firefighters) have fought a lot of fires in the past few months and are fatigued.”
Mr Concialdi said firefighters from 11 Western states are aiding firefighting efforts.
Crews are now trying to protect communities along the scenic coastline, as the blaze spreads from Ventura and Santa Barbara counties, about 100 miles northwest of Los Angeles.
California Department of Forestry and Fire Protection (Cal Fire) said: “Fire will continue to threaten the communities of Carpenteria, Summerland, Montecito and surrounding areas.”
Nearly 5,800 firefighting personnel are working on the blaze, Cal Fire said.
The cost of fighting as of Sunday was nearly £25million ($34m), the agency added. It is already the fifth-largest wildfire on record in California.
Some of the other fires burning over the past week in San Diego and Los Angeles counties have been largely controlled by the thousands of firefighters on the ground this week.
Both the Creek and Rye fires in Los Angeles County were 90 percent contained by Sunday morning, officials said, while the Skirball Fire in the posh Bel Air neighbourhood of Los Angeles was 75 percent contained.
North of San Diego, the 4,100-acre (1,660 hectare) Lilac Fire was 75 percent contained by Sunday and most evacuation orders had been lifted.
At the University of California, Santa Barbara, final exams set for this week have been postponed, Chancellor Henry Yang said in a letter to the campus community. Air quality and transportation issues, along with power outages that have affected the school’s information technology department, forced the delay of exams until January.
Santa Barbara County Sheriff Bill Brown tried to reassure the public they would be kept safe.
He told an evening press conference last night: “This is a menacing fire, certainly, but we have a lot of people working very diligently to bring it under control.”
Fire officials said they had managed to build lines between the towns of Montecito and Carpinteria to gain access to the flames.
But 5,000 people have been evacuated from their homes in both towns, which are near Santa Barbara and 15,000 homes are under threat.
The hot dry are blowing in from Santa Ana at 55mph were dubbed “devil winds” as they were expected to fan the flames heavily overnight.
They have not thought to have been as bad as anticipated, giving fire fighters a window to make progress.