Madrid Open star retires from opening match days after lifting trophy in Barcelona

's winning run came to an abrupt end on Friday night as he retired from his opening match at the Madrid Open. Just five days earlier, the world No. 9 upset Carlos Alcaraz to be crowned the But he couldn't carry his momentum into Madrid.

Rune received a bye into the second round of the Masters 1000 tournament and struggled in his match against Italy's Flavio Cobolli. He dropped the first set 6-2 before throwing in the towel. And the 21-year-old has now explained why he was forced to retire.

The Dane went on an impressive run in Barcelona last week, taking out defending champion Casper Ruud in the quarter-final before beating an ailing Alcaraz to lift the trophy and end his two-year title drought.

However, Rune could not repeat his heroics in Madrid, as his campaign was over almost as soon as it started. As the No. 8 seed, Rune received a bye into the second round to face Cobolli.

They took to the Arantxa Sanchez Stadium on Friday evening, but Rune spent just 42 minutes on court, losing the last four games of the first set and retiring with Cobolli leading 6-2.

The 21-year-old later took to social media to confirm that he picked up an injury earlier in the day and could not continue the match.

"Unfortunately I had a little twist in my knee during warm up today, and I felt it during the match," Rune tweeted

"I think it was the best solution to stop even though I was looking very much forward to continue my positive ride in Spain. See you next year @MutuaMadridOpen."

It's yet another setback in what has been a stop-start season for the world No. 9. When he's been fit, Rune has been able to show off the form that helped him become a Masters 1000 champion as a teenager at the 2022 Paris Masters.

But the Dane has also pulled out of multiple tournaments already. Back in February, Rune revealed that he had the flu after losing a one-sided match to Pedro Martinez in Rotterdam.

The world No. 9 suffered another and claimed he shouldn't have played so soon after getting ill, withdrawing from the Rio Open.

Rune was struck down by sickness again, retiring from his round-of-16 match at the Mexican Open after playing just three games. He later .

He had a promising run in Indian Wells, beating Stefanos Tsitsipas and Daniil Medvedev on his way But Rune suffered at the Monte-Carlo Masters, .

The five-time title winner now has less than two weeks to recover in time for the Italian Open, which kicks off on May 7. As one of the top players, Rune likely won't have to play his opening match until a couple of days later.