Young Indian shuttler Lakshya Sen on thursday clinched the Bulgaria Open International Series title after notching up a hard-fought win over Croatia's Zvonimir Durkinjak in the men's singles final in Sofia, Bulgaria.The 16-year-old shuttler defeated second seed Zvonimir 18-21 21-12 21-17 in a 57-minute match. He had beaten Sri Lanka's Dinuka Karunaratna 21-19 21-14 in the semi-finals yesterday.