What was the go with the missed break back opportunity in the 4th set tie break?
Del Potro serving at 4-2, first serve fault, second serve ball is long, someone in crowd calls 'fault', Federer bats it back casually and loses point. He then asks umpire, who confirms that linesperson didn't call anything. He then says "ok, can I challenge"? "yes" and challenge shows ball is long.
That should have been end of it. Double fault and 3-4 with Federer to serve. Del potro then mildly objects, umpire explains to both and then calls a 'let' and replays point entirely...both first and second serves. First serve fault again and second serve he wins point.
Three successive 'faults' and still gets the point...back to 2-5 on Federer's serve and loses tiebreak eventually.
Since when is a successful challenge not 'successful'?