The powerful topspin forehand, the signature shot which propelled him to 14 grand slam titles and made him virtually unplayable on clay, was deserting him at crucial moments.
With his movement tentative and his confidence at a low ebb, Nadal was humiliated by Novak Djokovic in the French Open quarter-finals -- a tournament he had ruled for a decade.
Lesser players also hunted him down, with many predicting the Mallorcan was stuck in an irreversible downward spiral.
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This week at the ATP World Tour Finals, however, Nadal has crushed Andy Murray and Stanislas Wawrinka, the world number two and four respectively. The instincts that allowed him to dominate the game seem to be alive and kicking.
"The sport in general is simple," Nadal told reporters at the O2 Arena, after his brutal 6-4 6-1 victory over Murray left him on the verge of reaching the semi-finals.
"When I hit the ball and I know the ball will go in, when I don't think about how to hit the ball, when I think just about where to hit it, that's good news.
"This year, I've been worried about how to hit the ball, not to miss, rather than hit the ball where I can damage the opponent. I have been more worried about myself than the opponents for a lot of matches. Like that it's impossible to compete at the highest level.
"Today I feel free. That means I'm happy."
Ranked back at five in the world, after slumping to 10th during a miserable summer which included defeats by dreadlocked German Dustin Brown in the second round of Wimbledon and Fabio Fognini at the U.S. Open, Nadal appears to be heading into 2016 ready to challenge for the big prizes again.
"I'm happy how the things are going the last couple of weeks," said the 29-year-old Nadal, who since his third-round exit at the U.S. Open has reached finals in Beijing and Basel and the semis at Shanghai -- a run that guaranteed his place amongst the elite at the London finale.
"It's good to finish the year with this positive energy. That helps to try to start next year in better shape."
(Reporting by Martyn Herman; editing by Rob Hodgetts)
