At the age of 15, Markešević set the 3000-meter national record for the 15-year-old age group with a time of 8:51.58. Markešević made his international debut at the 2003 European Youth Summer Olympic Festival in Paris, France, where he finished second overall in the 3000 meter race. At the 2005 IAAF World Cross Country Championships, Markešević finished in 36th place of 125 finishers in the junior men's 8K race. At the end of the same year, he finished in third overall in the junior men's race at the 2005 European Cross Country Championships. At the 2006 European Cross Country Championships, Markešević placed in second overall against an extremely competitive U23 field, finishing ahead of Daniele Meucci and Yevgeny Rybakov.