Sisi, who plays for London Irish, helped England to a 64-5 win against the country he could have opted to play for through his Italian father.
After the victory in Cape Town, the versatile back-row star tweeted: "Good win from the boys! Toughed it out at start,but great finish! Feeling it now, so recover, On to the next one! thanks for all support!"
Sisi and the rest of the England U20 squad take on Ireland in Stellenbosch on Friday.