Au fait and Up to it

Au fait adjective - Being up to particular standard or level especially in being up to date in knowledge.

Up to it and au fait are semantically related In some cases you can use "Up to it" instead an adjective phrase "Au fait".

Au fait and up to it are semantically related in able topic. Sometimes you can use "Au fait" instead an adjective phrase "Up to it", if it concerns topics such as able to perform, able to perform well.