"We have not reached our limits yet, this team still has room for improvement - that is a challenge. We can improve on an individual and team level," Kek said in an interview.
Slovenia, with a population of only two million, last week became the smallest country to qualify for the finals twice.
"This is the biggest victory of my career," said Kek, 48, who has been Slovenia coach for three years having managed the national under-16s and local team Maribor for seven years.
"A team game is my soccer philosophy, that was one of our biggest trump cards," said Kek.
He added that while he did not shake hands with Hiddink after Russia's defeat because of the commotion, the Dutchman has since sent his best wishes for next year's tournament.
"After the match it was difficult to find each other... He called me three or four days later and wished us success in South Africa," Kek said.
All Slovenia's current national team play abroad.
"Some credit goes to the players' clubs because our players were always well prepared," said Kek.
