On the 1.91-kilometre stage around the city’s Hadji Dimitar Stadium, Avci shined in dry conditions and in temperatures of 26 degrees.
The reigning Turkish champion, who claimed his maiden IRC win in Ukraine earlier this month, stopped the clocks in 2:00.5 at the wheel of his M-Sport Ford Fiesta S2000.
IRC Production Cup contender Marco Tempestini was second fastest, 0.5s adrift of Avci in his Subaru Impreza R4 STI, with category rival and IRC 2WD Cup pacesetter Robert Consani next up in his Renault Mégane RS, 3.4s down on Avci.
Cheered on by thousands of local fans, Dimitar Iliev, the five-time Sliven winner and favourite for victory in his SKODA Fabia Super 2000, was fourth fastest 4.0s, off the lead.
Former IRC 2WD Cup champion Harry Hunt was fifth, with Petar Gyoshev completing the top six in his Peugeot 207 S2000.
Menya Krozser (Renault Clio R3) was seventh, followed by Bulgaria’s Ekaterina Stratieva and impressive IRC rookie Yuan-Hu Lin from Taiwan.
Murat Bustanci completed the top 10. Rising Bulgarian star Todor Slavov was 11th quickest with Martin Kangur the fastest Honda driver 15th overall.
The stage was stopped for a time after Bulgarian IRC 2WD Cup contender George Lazarov crashed into a tree. Lazarov was taken to hospital for precautionary checks but was subsequently given the all clear.