There were less than five minutes to go when the impressive Mercer scored his second try in a match which the England under-20 team, bidding for a fifth consecutive final appearance in the competition, dominated initially before the Baby Boks fought back.
It has to be said that Mercer broke through some woeful tackling on the way to the score.
The South Africans were down 17-7 at one stage but yet another try to their equally influential No 8 Juarno Augustus brought them back into it on the stroke of halftime as England went to the break 17-12 up.
