SB 7.3.20

vyavasāyena te ’nena
 duṣkareṇa manasvinām
tapo-niṣṭhena bhavatā
 jito ’haṁ diti-nandana
vyavasāyena — by determination; te — your; anena — this; duṣkareṇa — difficult to perform; manasvinām — even for great sages and saintly persons; tapaḥ-niṣṭhena — aimed at executing austerity; bhavatā — by you; jitaḥ — conquered; aham — I; diti-nandana — O son of Diti.
My dear son of Diti, with your great determination and austerity you have done what was impossible even for great saintly persons, and thus I have certainly been conquered by you.

In regard to the word jitaḥ, Śrīla Madhva Muni gives the following quotation from the Śabda-nirṇaya: parābhūtaṁ vaśa-sthaṁ ca jitabhid ucyate budhaiḥ. “If one comes under someone else’s control or is defeated by another, he is called jitaḥ.” Hiraṇyakaśipu’s austerity was so great and wonderful that even Lord Brahmā agreed to be conquered by him.