career wise in last decade besides Nalbandian is 100% Blake
Blake hit so insanely hard and good (my god this is sounding strange now
) at times in the mid 2000s
frankly if you watch the best highlights of BLake, ala that US Open match against Agassi 2005, it makes today's game look fairly weak, they were just pounding the ball going for winners
granted maybe possibly even US Open 2005 court was a bit more favorable to faster play and also Blake-Agassi not being THAT defensive.
either way Blake achieved so little for so much talent.
he could hit winners out of nowhere like no one else. unfortunately now his movement is atrocious so he stands no chance to make a comeback, although he seems to think otherwise for some strange reason
as far as YOUNGER players,
certainly Del Potro has underachieved but that's partially due to bad luck
Dimitrov. for all his talent has underachieved even if he's still very young
Cilic.... let's not even start there....
Baghdatis, althoughe he is getting older too, and it's widely known that he underachieved due to lack of focus and a bad unhealthy lifestyle
you could honestly come up with so many talented players who are and have 'underachieved' because there are so many gifted players as the sport is so worldwide now