Sadly Del Potro already lost this match without even playing it. He is not convinced that he has the game to defeat Nadal. If he goes in with the right attitude the it may be a close match, otherwise...
this is confusing because, as they say, you can't have it both ways.
#1 you say delpo's mental attitude is so bad that he might as well just roll over ("He is not convinced that he has the game to defeat Nadal" and that he has "already lost without even playing the match")
#2 and then you turn around and say the opposite, namely, that, "If he goes in with the right attitude the it may be a close match," when you've already stated he does not have the right attitude.
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My opinion, having seen all of delpo's miami matches, is that delpo is brimming with confidence in himself and his results here so far, and he has every right to that confidence. every one of his 3 opponents in miami thus far has been picked by someone(s) to beat Delpo, and none of them has--not because they were so inferior to Delpo, but because on the day that they played him, none of them could solve him.
One of the things I love about nadal is that he gives each opponent respect and plays them as such. nadal will not discount anything about delpo, but he knows that delpo w/b playing him in his house, for starters. yes, stadium court is rafa's house. delpo has played one match there, while rafa has played all of his this year plus all of his last year, so that's one advantage for rafa.
another advantage for rafa is that delpo is younger and less experienced. delpo is still feeling his way, gaining the experience that rafa already has after 4 more years of experience. delpo turned pro in 2005 (4 years ago), while rafa turned pro in 2001 (8 years ago), so rafa's been on the professional tour twice as long as delpo has.
rafa is more mentally and physically developed than delpo, but delpo has been in the zone in miami/key biscayne. World No. 7 will give World No. 1 a better match than he did at IW a couple weeks ago, IMO. all he needs to do is play within himself, and just at the moment, that's a good place to be.
none of delpo's opponents has been able to take him out of his zone in the week since March 28, whereas both gabashvili and wawrinka have been able to take rafa out of his zone.