Britain’s Emma Raducanu said she was "quite tired and lethargic" during her 6-2 6-1 loss to Marta Kostyuk in the Transylvania Open quarter-final.Ukraine’s Kostyuk overcame her opponent in just under an hour at the WTA 250 event in Cluj-Napoca. Raducanu looked out of sorts during the match, committing 41 errors.The 18-year-old seemed understandably disappointed with her performance."I wasn't physically feeling 100 per cent," she said. "I was quite tired and lethargic today. Sometimes you just have those days where you don't feel your best. I knew from the morning, I knew from practice."I wanted to go out there on the court and try my best and see how it was going to go, maybe it would go better. But I just couldn't get it going today unfortunately."It's just disappointing that you have these days. It's not a nice feeling to have, but I just need to move on from it and brush it off. And then I'll be back in another tournament soon."

Raducanu is scheduled to play at Upper Austria Ladies Linz next month, and will also appear at an exhibition event at the Royal Albert Hall in London.

She suggested her recent lack of energy was down to the intensity of the past few months, which has seen the teenage tennis star shoot to international stardom after winning the US Open.

"I think it's just the last six months," Raducanu said. "It's been a lot of learnings and I've experienced a lot in the last six months with not so much gaps.

"I'm just adapting to the fast-paced life of the tour and obviously still very new to it, so it's still going to take me some time to adjust.

"After Linz I'll probably have a week off just to reset and [be] mentally and physically fresh for the tough preseason. It's going to be my first one and from what I've heard, it's a very difficult four weeks physically."

Emma Raducanu was disappointed with her performance at the Transylvania Open

Raducanu appeared wary going into her match against Kostyuk, claiming she had "destroyed" her "every single time" when they used to play each other at a junior level.

The 19-year-old Kostyuk, five months older than Raducanu, seemed to be spurred on by this, telling reporters afterwards that it "gave me some strength".

Raducanu earned her first victory on the WTA Tour when she defeated Polona Hercog in the opening round of the Transylvania Open, before overcoming home favourite Ana Bogdan.

She will now miss out on a blockbuster semi-final clash against idol Simona Halep, who will take on Kostyuk instead.

Raducanu became the first qualifier, male or female, to earn a Grand Slam title at this year’s US Open.

She also became the first British woman to win a Grand Slam title since Virginia Wade in 1977. Raducanu made history in style, not dropping a single set on her way to victory.