Carlos Alcaraz has withdrawn from the Barcelona Open due to a right wrist injury.
Alcaraz, who needed to win the tournament to regain the world No 1 ranking, and was set to face Tomas Machac in the second round.
A two-time champion in Barcelona, Alcaraz had returned to competition just 48 hours after losing the Monte-Carlo Masters final to rival Jannik Sinner.
The Spaniard trained at the Real Club de Tenis Barcelona on Wednesday but decided to pull out after undergoing a test on his right wrist.
Speaking to the press, Alcaraz said: "The truth is that it's strange and difficult to sit here for the second time, because I've done this before, and to say that I won't be able to continue in the tournament.
"As everyone could see yesterday in the match, after a return I felt like my wrist gave out and I started to feel discomfort that, little by little, kept increasing during the game.
"It's a situation that I thought I'd felt before and that wouldn't get worse, that it was simply discomfort from the effort of the entire week, but I saw today's exams and it's an injury a bit more serious than everyone expected.
"And the truth is that I have to listen to my body, listen to what's best for me and won't harm me in the future. And that's why I have to withdraw from this tournament, which for me has always been an exceptional tournament, a wonderful tournament, a super special tournament.
"Personally, I never like to give up on any tournament, but this one is especially important to me.
"I have to go back home to start recovery as quickly as possible with my team, with the doctors, with the physiotherapist, and try to be as healthy as possible, as quickly as possible, for the tournaments I have in the future.
"I hope you can see me back on a tennis court as soon as possible."
Alcaraz faces a race to be fit for Madrid and Rome as he builds towards the defence of his French Open title, which begins in Paris on May 24.
British No 1 Cameron Norrie booked his place in the quarter-finals in Barcelona after coming through a tough three-setter against American Ethan Quinn, a match which lasted two hours and 32 minutes.
After Norrie took the set 6-3, his 22-year-old opponent levelled it up in the second 6-4 to take the match into a decider.
But Norrie, 30, then took the final set 6-4 to book what appears likely to prove a competitive quarter-final tie on Friday against Spanish 19-year-old Rafael Jodar, who is yet to drop a set on home soil.
Jodar, the world No 55, is on a run of seven straight wins after claiming his maiden ATP Tour title at the 250 event on the clay in Morocco earlier this month and beat Norrie in their one previous meeting at the Mexico Open in February.
On the WTA Tour, Katie Boulter also progressed to the last eight after knocking out third seed Jaqueline Cristian on the clay courts at the Open de Rouen in France.
Boulter, the British No 3, beat her Romanian opponent 7-6 (7-5), 4-6, 6-1.
And at the WTA's 500 event in Stuttgart, the Porsche Tennis Grand Prix, third seed Iga Swiatek secured a quarter-final berth by winning her first match of the year on clay 6-2 6-3 against Germany's Laura Siegemund.
It was the four-time French Open champion's first match with Francisco Roig as her coach after his split from working with Emma Raducanu.
"Even though I'm an experienced player, it's still a new thing for me," said Swiatek.
"I haven't changed coaches often in my career, but I feel it's always exciting because you get to know different points of view from a person that is supposed to be with you every day on the court.
"So I think you need to be open-minded and try to soak in this new approach. And I feel like with Francisco, we have a similar view in terms of how I should play. The other thing is actually doing it on court. So this will, I think, take a little bit more time, but the idea is there, the attitude and work are there, so I'm happy to start this process."
Novak Djokovic has announced he will return to action at the Mutua Madrid Open - live on Your Site Tennis from April 21 to May 3.
The Serb is a three-time titlist, last lifting the trophy in 2019, but has not competed on the ATP Tour since losing to Britain's Jack Draper at Indian Wells on March 12.
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