"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.
They beat Russia 1-0 in the play-offs to go through on away goals following a 2-2 draw on aggregate.
Slovenia, which declared independence from the former Yugoslavia in 1991 and joined the European Union in 2004, made their debut in the World Cup finals in 2002.
The victory over Russia was the biggest surprise in the playoffs and prevented Dutch coach Guus Hiddink guiding a team to a major tournament for the first time in his career.
"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.
TEAM SPIRIT
Kek, who declined to predict how far Slovenia could go in the finals, said their ability to beat Russia was based on team spirit which is also their best hope for future success.
"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.
He added that the globalisation of the game increased the chances of success for smaller nations at the major tournaments because their players have joined the best clubs.
All Slovenia's current national team play abroad.
"Some credit goes to the players' clubs because our players were always well prepared," said Kek.
He added that the biggest challenge before the World Cup will be preparing for conditions at the tournament, namely the temperature and altitude. Slovenia will begin their logistical arrangements immediately after the December 4 World Cup draw.
