Del Potro is not so mentaly strong. He has a weapon that works always for saving bps and match points- his serve.
When you have great serve its easy to save those, while in play everything can happen.
he brings his best serve at the most crucial moments when its now or never.
for example the final in washington last year against roddick, in the last set tie break he hit around 4/5 aces, whereas he'd hit around the same in the rest of the set. im not sure about the exact figure so this is just a rough estimate. voo should know this tho
anyway, if u had any logic you would see this shows hes incredibly strong mentally, and probably mentally toughest player atm.
- double faulting twice in the US open finals against federer to hand him the set, yet carrying on and winning the match.
- breaking federer at the last oportunity possible to get through to the finals at the WTF in london.
- being down 0-3, two breaks of serves against nadal in Miami in the final set, and winning the match in a tie-break.
you get the picture.