American teenager Coco Gauff is through to her first career WTA 1000-level semifinal at the Internazionali BNL d'Italia after World No.1 Ashleigh Barty retired in the second set of their quarterfinal match.

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On a day which saw a nearly two-hour rain delay in the first set, Barty was leading, 6-4, 2-1, with Gauff to serve, before retiring with a right arm injury during the changeover.

The World No.1 played the match, her 16th in singles so far in the clay season, with wrapping on her left thigh, and a compression sleeve on her serving arm, telling reporters after the match that the injury has been a recurring problem in her career. 

"It became worse while we were playing," Barty said of the pain in her arm. "So I think that's the challenging thing is to make the decision to stop. It's never nice. It's the thing that I hate the most is not being able to finish a tennis match.

"But the pain was becoming too severe, so it was important that I listen to my body and of course try and do the right thing, knowing that we have a Grand Slam in two weeks' time."

It is Barty's first retirement in nearly three years, when she retired in the second set of the semifinals at the 2018 Internationaux de Strasbourg.

"I hate withdrawing. I hate pulling out of a match halfway through. It's not in my makeup, not what I like to do, but it was really important today that I listen to what my body was telling me to make sure that in a couple weeks' time we're good to go," Barty said.

"It's something I have had to manage over my career. It's an injury that began when I was quite young, when I was 15 or 16 years old. It just pops up every now and again.

"I think the conditions today certainly didn't help that, but I mean, we're confident we know how to manage it, so we just kind of move on and know that the right decision was made today, as hard as it is."

In what was a high-quality match in wet and heavy conditions prior to Barty's retirement, the World No.1 hit 20 winners to 15 unforced, and broke serve twice. She was a break down in the opening set at 2-1, but immediately restored parity and broke to wrap up a one-set lead on her fourth set point. 

"When she called the physio, I mean, I knew she had her leg taped before the match, but I think she always has that taped... When she came up to me, I was shocked. I didn't know what to make of it, just because I didn't feel like there was anything going on," Gauff said. 

"It's not a way you want to win a match. At the end when she told me she was retiring, forfeiting or retiring the match... Obviously I felt for her, but when you have the chance to play the No. 1 player in the world, it's not a good feeling to win this way.

"I send the best well wishes, and hopefully she can get well before the French Open."

Gauff posted her own similar statistics, hitting 12 winners to 13 unforced, and earns abridged passage through to her four WTA singles semifinal overall. The 17-year-old's run this week assures her a ranking rise into the Top 30. 

"I'm very happy, especially with my first three matches. Those wins were great. It's cool to make the semifinals," Gauff said.

"I felt like today, even though the first set didn't go the way I wanted, I felt like I was bringing the energy from the start after every break. I think that's something I think I did well today."

Up next, Gauff will face either Elina Svitolina or Iga Swiatek for a place in the final. The final quarterfinal match was postponed due to rain, and will be played Saturday ahead of the semifinals. Gauff has only faced Svitolina: the Ukrainian was a 6-4, 6-3 victor in the second round of the Australian Open.

"I played Svitolina in Australia. Honestly that match, even though the score didn't say it, I felt like it was a high-quality match. She plays super well, so I know what I have to expect, high intensity from the first point to the last," Gauff said. 

"Regarding Swiatek, she has great results on the clay. Obviously, she won French Open last year... so I think either/or it's going to be a tough match. They are both fighters. They both have good results in Grand Slams and in 1000s, so I think it's just going to come down to who wants it more. I'm going to try my best."