Alexander Zverev has admitted he is at a loss to explain his dip in form after the world number three was beaten by lucky loser Nicolas Jarry in the second round of the Barcelona Open on Tuesday.
Zverev spurned a match point at 6-5 and then a 3-0 lead in the final-set tie-break before going down 3-6 7-5 7-6 (7/5) to Jarry, ranked 81st in the world.
The German had taken a late wildcard in Barcelona in search of both matches and form after a string of surprise defeats.
"I just play bad, it's not a secret,” Zverev said afterwards.
"The tennis was very low-level. Most of the points that I won were unforced errors by him. I'm in a hole and I don't know how to get out of it."
Jarry had even lost in qualifying to Spain's Marcel Granollers but, after being given a reprieve, knocked out the same opponent in the first round to set up a meeting with Zverev, long-considered a future star of the men's game.
The Chilean upset both Marin Cilic in Shanghai and Dominic Thiem in Hamburg last year but this was the biggest victory of his career and another setback for Zverev, with just over a month to go before the start of the French Open.
The 22-year-old beat Roger Federer and Novak Djokovic en route to winning the ATP Finals at the end of last year but has since struggled to maintain momentum.
He lost in the fourth round at the Australian Open in January and has now failed to win more than a single match for a fifth tournament in a row.
Jarry, meanwhile, will face either Bulgaria's Grigor Dimitrov or veteran Spaniard Fernando Verdasco in the third round on Thursday, with Japan's Kei Nishikori now the highest ranked seed left in the bottom half.
Nishikori, Barcelona champion in 2014 and 2015, has also been out of form in recent weeks but recovered from a slow start to beat American Taylor Fritz 7-5, 6-2.
The world number seven will be up against either Canadian Felix Auger-Aliassime or Tunisia's Malek Jaziri for a place in the quarter-finals.
Top seed Rafael Nadal begins his bid to win the Barcelona Open for a record 12th time against Argentine Leonardo Mayer on Wednesday.