His strongest surface over the course of his career, was indeed hardcourt, where he won six of his eight majors.
hɪz stɹˈɔŋɡɪst sˈɜːfɪs ˌoʊvɚ ðə kˈoːɹs ʌv hɪz kɚɹˈɪɹ, wʌz ˌɪndˈiːd hˈɑːɹdkoːɹt, wˌɛɹ hiː wˈʌn sˈɪks ʌv hɪz ˈeɪt mˈeɪdʒɚz.