Why did Par'oh not ask Moshe and Aharon to leave (as he did earlier in 5:4)?
Ramban (in 5:4): This time, when Moshe and Aharon responded to Par'oh's request for a sign, by proving themselves (at least) on a par with his magicians, their esteem rose in his eyes, and he now considered them equal to his wise men. 1
Ramban: Moreover, he feared the imminent Makos; and from the moment the Makos began, he became awestricken before Moshe and Aharon. See however, Bo 10:28.