This study aimed to develop an in-situ sludge pretreatment method by increasing the temperature from thermophilic to extreme thermophilic condition in a single-stage anaerobic digester. The results revealed that a stable performance was obtained within the temperature range of 55-65 degrees C, and the maximum methane yield of 208.51 +/- 13.66 mL/g VS was obtained at 65 degrees C. Moreover, the maximum extent of hydrolysis (33%) and acidification (27.1%) was also observed at 65 degrees C. However, further increase of temperature to 70 degrees C did not improve the organic conversion efficiency. Microbial community analysis revealed that Coprothermobacter, highly related to acetate oxidisers, appeared to be the abundant bacterial group at higher temperature. A progressive shift in methanogenic members from Methanosarcina to Methanothermobacter was observed upon increasing the temperature. This work demonstrated single-stage sludge digestion system can be successfully established at high temperature (65 degrees C) with stable performance, which can eliminate the need of conventional thermophilic pretreatment step. (C) 2017 Elsevier Ltd. All rights reserved.