What American male singles player [ATP] has the most impressive and encouraging 2015 results so far?

Jack Sock, age 22

68%

John Isner, age 29 [turns 30 Apr. 26]

26%

Donald Young, age 25

5%

Sam Querrey, age 27

0%

Steve Johnson, age 25

0%

Tim Smyczek, age 27

0%