I mentioned the test of not being able to read more than a couple pages of text without losing concentration.

我曾提到过，通过关注自己是否能够集中注意力去阅读一两页书来判断自己是否已经困倦了。

youdao

"To be in the quarter-finals of the US Open without losing a set and without losing a serve, two things must work really well - the concentration and the serve, " said Nadal.

BBC: Rafael Nadal beats Feliciano Lopez at US Open