Rafael Nadal admitted that it will be "hard to return to the practice court" after a shock semifinal defeat by Fabio Fognini ended his bid for a record-extending 12th Monte Carlo Masters title on Saturday.
The 17-time Grand Slam champion starts his Barcelona Open title defence on Wednesday, as he also fights to keep his world-number-two ranking before the French Open gets underway next month.
The 32-year-old said he played "one of my worst matches in 14 years on clay" in losing 6-4, 6-2 to Italian Fognini, having been forced to save three match points at 5-0 in the second set.
"It will be hard to return to the practice court," a subdued Nadal said. "I lost an opportunity in a very important event for me. I'm sad for that."
Nadal has struggled with knee injuries this season, and was playing his first event since withdrawing before an Indian Wells semi-final against Roger Federer last month.
"There was little chance for me to start the clay-court season in a perfect way," he insisted. "It was a bad day, the kind of day where the feeling is not there at all. It's difficult to find positive ways today."
"I was lucky to win two (second-set) games to avoid an even worse score," he admitted. "I'm thinking about how bad I played today, (trying to) understand the things. Coming back from injuries, winning and winning and winning is not easy.
"(It's been a) year and a half of not having the chance to play tournaments in a row (due to injury)."