posustati

1. to tire; to flag; to falter
   Na poslednjem kilometru je posustao i stao. — On the last kilometre he flagged and stopped.
   Nije posustala ni kad joj je bilo najteže. — She didn't falter even when things were hardest.
2. (in a race, contest) to fall behind; to lag
   U borbi za titulu tim je posustao. — The team fell behind in the title race.

Often used with negation: «nije posustao» — he didn't give up or weaken.