I remember pondering in the R16 which of the 4 potential candidate would be the biggest challenge for Nadal:
Delpo
Cilic
Fognini
Isner
The more I thought about it, I could make a case for any of them.
Delpo's A game is the best of the 4.
Cilic has a win in a major against Nadal (no less, the very last major played!).
Fognini has beaten Rafa on clay multiple times, is completely not bothered by Rafa's heavy topspin (due to ability to take the ball early) and won 5 straight games against him just a few weeks ago.
Isner, while the only one of the 4 to never beat Rafa, took peak Rafa to 5 at RG as a far inferior player in 2011, took him the distance at Monte Carlo, and lost a tight 75, 75 match in a Masters final. With his serve, he's tough enough to break that he could win a coule TBs and make it interesting.
Still, I don't know which of the 4 I'd take. If really pressed on it, I might lean Fognini, but Delpo certainly will have his chances, especially considering Rafa's average form.