Intention in prayer

Dear Rav, I had recently asked a rabbi if the halacha was in accordance with the opinion of Rav chaim Brisk who maintains that throughout the Amida one must have the awareness of standing before G-d. The answer was negative . Nevertheless, the rambam writes explicitly that it is necessary to have such kavana during tefila. So I deduce that it is not necessary to have this kavana throughout tefila but only at the beginning. Does this mean that he who did not think he was standing before Gd even at the beginning of the Amida he didnt fulfill the mitzvah? On the other hand, if it is said that everyone who prays necessarily has the awareness that he is before Gd, then why did the rambam need to specify in his alachot that he must to have such a kavana? Thank you very much