1) Justine is little Serena

2) Justine is a 21 years old Conchita Martinez with a big serve.

Now, let's try to put this in mathematic terms.

What "little" means here? little by age? no, because they are about the same age and I don't think it means Serena as she was when she was "little", so I think little here means small, and if you say small you mean smaller, we can say

S = J * x

where

S = Serena

J = Justine

x = a factor of how much "smaller" Justine is.

Now, the second definition.

a 21 years old Conchita with a big serve. Again, we can use a factor to weight the "21 years old part", I'll call it y and we take out Conchita's serve and add Justine's serve

C = (y * J) - CS + JS

where:

C = Conchita

J = Justine

CS = Conchita's serve

JS = Justine's serve

y = age factor.

if we work this equations we have

1) J = S / x

2) J = (C + CS - JS) / y

wait a moment, we have two J, so we can say

S / x = (C + CS - JS) / y

S = x/y * (C + CS - JS)

what does this tell us?

Serena is Conchita minus Justine's serve + her own serve, multiplied by the relation between Serena and Justine's body and divided by the relation between Conchita and Justine's age.