Emilio Sánchez Vicario, the former leading player and Spanish Davis Cup captain, said in an interview Sunday that after Nadal surprised the opposition with his quick-strike, higher-risk approach in 2013, he is struggling this time to switch from hardcourts to clay. Sanchez said he also believed that Nadal’s backhand had become less reliable because of a change in footwork presumably designed to protect his fragile left knee.

“He needs to keep playing the same way because we don’t know how much he still has difficulties with the legs and where to put the weight,” Sánchez said. “But what was his best before, the defense, he has to improve it today to be less erratic. Last year he was doing it with the drive and the passion and will to become No. 1 again. Now he is No. 1 and has to find those things again to not give the opportunity to the opponent.”

Nadal’s genius has been his rare blend of tennis savvy and tennis amnesia. “It’s like what just happened doesn’t matter,” Annacone said. “All that matters is now, this second, and the next four seconds.”

But Nadal also tends to see danger at every turn, which can become, in periods of struggle, a self-fulfilling prophecy. “I don’t want to say it’s disingenuous but it’s almost like an unrealistic level of humility,” Annacone said. “Psychologically his approach helps him to be ready for every opponent and every point. That sense of urgency is so admirable, but I have to say it’s incredibly exhausting.”

Annacone does not buy the theory that Nadal is struggling to make the shift from hardcourts to clay. “What I do buy is a lot of the stuff that could be mentally or emotionally related,” he said.

The comeback from injury to No. 1 in 2013 had to be draining, just as his back problem and surprise loss to Stanislas Wawrinka in the Australian Open final this year had to be deflating as well as encouraging to the opposition. And so it might be reassuring for Nadal at this shaky stage to know that he will not have to worry this week about Djokovic, who overwhelmed him in the Miami final in March but withdrew from the Madrid event Sunday because of concerns about his right wrist.

The uncertainty about Djokovic’s health further clouds a season in which it has been difficult to see through the mists. Andy Murray, after a back operation, has not been the same player who won Wimbledon last year. Juan Martín del Potro is out of action for an extended period because of a wrist operation. Federer has been resurgent and impressive but is not yet back to vintage Federer. Djokovic just announced that his fiancée, Jelena Ristic, was expecting their first child.