According to this MA thesis, the Byzantine armies had a number of overall advantages, but it does not follow that their commanders were especially skilled at one-to-one combat. Forcing individual Byzantine commanders to fight duels was an effective Arab strategy to exploit this weakness. (EDIT: As mentioned by @MarkC.Wallace in the comments, the Mubarizun were an Arab unit that specialized in single combat. It appears that that the Byzantines may have lacked any similar specialization.)
Specifically, at the Battle of Ajnadayn:
it was the Byzantine commanders who had the greater tactical and
strategic expertise; thusly al-Walid chose to weaken the Byzantines
using their own vanity as a weapon. The Muslims’ individual champions
would march out to the fore and challenge the Byzantine officers to
single combat. Unfortunately for the Byzantines, this ploy worked.
Scores of experienced Byzantine officers were killed by the more
martial and battle-tested Muslim officers.
One of the prominent Arab champions who defeated numerous Byzantine officers in that battle and others was Dhirrar Ibn al-Azwa.