Del Potro has 15 points from Viña, but replace it with his 19th result and you'll have 9 points less. Then replace Costa's points with his 20th result and you get 5 points less (anyways he'll have his Memphis points by then).
You are right about Memphis, I didn't know the ATP had increased the amount of points this year (last year, a player received 5 points for a 1st round and 15 for a 2nd).
And how is Del Potro losing Costa do Sauipe's points if the Brazilian tournament last year was held in the 8th week (the week when Memphis is played).
Buenos Aires and Costa changed weeks.
As I see it Del Potro will lose Buenos Aires and Viña's points and going to defend Costa do Sauipe's points in Memphis.
I never said he was losing Costa's points in the 7th week, I just omitted Buenos Aires.