a small lapse on the left side

Kazuo Ishiguro氏のThe Buried Giantを読んでいます。
Even the best of us would have feared to meet him as a foe. Yet yesterday, when I saw him meet Brennus’s soldier in combat, I might have seen a small weakness on his left side. Or was it his clever ploy of the moment? If I watch him fight once more, I will know better. A skilful warrior all the same, and it would take a knight of Arthur to suspect it, but I thought it so, as I watched the fight. I said to myself, look there, a small lapse on the left side. One a canny opponent might just exploit. Yet which of us would not have respected him?
"lapse" は「経過(する)」の他にも「無効(にする/なる)」の意味があるのを10/24/2012に取り上げましたが、上の引用個所の "lapse" はさらに別の意味のようです。辞書を引きます。 ・Oxford English Dictionary: A weak or careless decline from previously high standards.: ‘tracing his lapse into petty crime’
・Macmillan Dictionary: a gradual or temporary change to a worse or more unusual type of behaviour or activity
左側に何か弱みがあると解釈しましたがどうでしょうか？