This paper addresses the nonlinear amplifications in the upwelling crests of air gap responses from the wave basin model test of a deep draft semi-submersible in extreme wave condition. Contributions from nonlinear incident waves, vessel motions and wave-body interactions are analyzed separately. Results from the analyses suggest that, nonlinear amplification factors are larger than predictions from second order corrected model for incident and diffracted waves. Low frequency roll and pitch motions will induce larger nonlinear negative vertical motions for points in down-wave area, thus worsen air gap performance. Upwellings are highly related to diffracted wave elevations. Local run-ups due to highly nonlinear wave-body interactions around column walls could result in the nonlinear amplification factors in this area to be up to 230%.