expression
don't count your chickens before they're hatched; there's many a slip 'twixt the cup and the lip
Proverb warning that the final stage of a task is often the hardest, and one should not become complacent before completion. Literally: 'One who travels 100 ri considers 90 ri to be the halfway point.'
「百里を行く者は九十を半ばとす」というから、最後まで気を抜くな。
As the saying goes, 'He who travels a hundred ri counts ninety as halfway,' so don't let your guard down until the very end.
A classical Chinese-derived proverb (from the 'Zhan Guo Ce' or similar texts), adopted into Japanese. The literal meaning uses the old Japanese distance unit 里 (ri), roughly 4 km.