The 24 year-old rising star, who unified the minimumweight division before moving up to claim his current belt, improved his record to 13-0 in the process with his ninth career stoppage as former world 105lb title holder Sithmorseng, 29, dropped only a second loss in 46 contests.
In the Osaka co-feature, Ryo Miyazaki also kept up his unbeaten run with a majority decision verdict over Mexican challenger Jesus Silvestre in what turned out to be the fight of the card and could conceivably gone either way.
Miyazaki, 25, battled his way to a 115-113, 115-114, 114-114 victory on the three judges’ scorecards to improve to 20-0-3, whilst 23 year-old Silvestre lost for the fourth time in his 31 bouts and can come again after a putting up another good showing.
Also on the bill, recent world bantamweight title challenger Malcolm Tunacao predictably saw off two-fight novice Imron Lookkhlongtan comprehensively via a five round technical decision 50-43 (all three) and home favourite Sho Ishida scored a second round KO to move his impressive resume to 14-0 with seven of those wins coming inside the distance since his 2009 debut.
