Heslop took third in the event at the stunning historic venue to keep alive his hopes of winning the series title.
In the five series events so far, Heslop has claimed victory in two and finished third in two. The only round he did not make the podium was in Paris.
He will go into the final leg in New Zealand 133 points behind leader Constantin Popovici, who was fifth in Mostar.
“I think it would take a little bit of a miracle for me to win, now that Costa came fifth in this event,” admitted Heslop. “But we’ll see. Anything can happen, we’ll just have to wait and see, do my best in New Zealand.”
The final round will take place on November 19.
In Mostar, where 17,000 people turned out to watch, Heslop was just edged out by Spanish wildcard Carlos Gimeno and Romanian Catalin Preda.
Entering the final round in the lead for the first time in his career, 33-year-old Gimeno held his nerve to reel off a spectacular back quint, demoting Romanian Catalin Preda into second place and Heslop into third.