Making his first appearance since recovering from an abdomen injury (and celebrating the birth of his first child), Nadal fell to a surprise 3-6, 7-6(4), 6-1 to America's Tommy Paul.
Having started the season in blistering fashion by winning both the Australian Open and French Open and reaching the Wimbledon semi-finals, the Spaniard has seen the latter half of the year derailed by injury and the demands of his personal life.
It's all combined to make Nadal feel less than optimistic about his chances at the season-ending ATP Finals - especially considering it's a tournament he has never won before.
Nevertheless, he once again confirmed he will be heading to Turin to give it his best shot.
"I'm excited about playing, even if it hasn't been the perfect couple of months for me," Nadal said.
"It's true that for the last five months I didn't spend enough days on the Tour. I don't even say competing on a tennis court. I say on the Tour. Practicing with the guys. That's what I need ...
"For me (it's) difficult to imagine now arriving in the World Tour Finals in enough good shape to win a tournament that I didn't win during all my tennis career."
Commenting on his loss to Paul, Nadal refused to make any excuses for his poor play, which saw him get broken three times in a one-sided final set.
"It's okay, no, all the credit to Tommy," said Nadal when quizzed on whether his injury was responsible for shaping the result. "Things that happens sometimes. But no, it's good, no? I think he played aggressive, a lot of great shots.
"I had my match in that second set with set and break. I played terrible game there. Yeah, I didn't deserve the victory playing that bad in that key moment, no.
"Until that moment was okay, a good match for me. Knowing that is my first match since a while. Then this surface especially you cannot make mistakes with your serve."