Ezenkosi beat Wits 5-4 on penalties after Mntambo scored in a 1-1 draw in regulation time at Tsakane Stadium.
"They [Wits] played their second-choice team," Mntambo tells KickOff.com.
"We knew they will lack match fitness. But our determination to reach the Nedbank Cup's last-eight for the first time helped a lot as well. We have never got past the last-16 stage of this cup but this time we did it. We are taking it, it's possible. Self-motivation was our secret yesterday.
"Our game plan was clear, all we wanted was to reach the half-time break with a 0-0 scoreline because we knew they wouldn't make it in the second half due to their players' lack of fitness."
Mntambo says now their focus is on their league match against NFD leaders Thanda Royal Zulu.
"Play-offs are very important, [more] than the cup. Our next game is against Thanda. In the first round they beat us 2-0 because we were one player short due to the red card we received that day. If we can reach the play-offs, I promise everyone that nothing will stop us in realising our dream of going back to the PSL."