Murad returned from Anatolia in 1388 and launched a lightning campaign against the Bulgarian rulers Shishman and Sratsimir, who swiftly were forced into vassal submission. He then demanded that Lazar proclaim his vassalage and pay tribute. Confident because of the victory at Plocnik, the Serbian prince refused and turned to Tvrtko of Bosnia and Vuk Brankovic, his son-in-law and independent ruler of northern Macedonia and Kosovo, for aid against the certain Ottoman retaliatory offensive.

The Battle of Bileća was fought in August 1388 between the forces of the Kingdom of Bosnia led by Grand Duke Vlatko Vuković, and the Ottoman Empire under the leadership of Lala Şahin Pasha. The Ottoman army broke into Hum, the kingdom's southern region. After days of looting, the invaders clashed with the defending force near the town of Bileća, north-east of Dubrovnik. The battle ended with an Ottoman defeat.