That is the verdict of Sky Sports tennis pundit Annabel Croft.
After going a set down, Federer levelled up the match with a tie-break.
However, the Swiss legend lost his way and after a final set tie-breaker, he ended his chances of a record sixth Indian Wells crown when he threw in two double faults.
The 36-year-old still retained his world No 1 spot despite the defeat, however his 17-match winning streak came to an end at the hands of Del Potro.
The Argentine, a former US Open champion, is now up to sixth in the world rankings.
And Croft thinks Del Potro was fired up when Federer hit a half volley that hit him in the legs as well as the misjudged drop shot with the game in control.
Discussing Del Potro’s win, Croft told Sky Sports: “The wind went out of Federer’s sails in the third set.
“Del Potro was absolutely fuming when he lost that second set.
“He came down the end where we were sitting and he was bouncing balls into the stock netting.
“He was so angry and wound up and was teeing off on forehands.
“It’s almost as if he would take out his frustration on his forehand.
“He looked a little bit tired and jaded and legs had gone a little bit.
“He kept the scoreline tight until he got broken.
“Federer made a couple of strange decisions at 40/15 – the drop shot that didn’t reach the net and when he hit the ball against Del Potro – he was so angry and fuming and that upped the level and got him into the tie-break which helped him close it out.”