Nick Kyrgios has lamented the ATP Tour trialling off-court coaching, warning tennis will lose one of the "unique traits that no other sport had".

The ATP announced on Tuesday that off-court coaching will be tested in the second half of the season, with coaching permitted by a designated person in qualifying and main draw matches.

Verbal coaching will be permitted when players are at the same side of the court as their coach, with non-verbal instructions – for example hand signals – allowed at any time.

Patrick Mouratoglou coached former world number one Serena Williams and now works with Simona Halep, and was quick to welcome the introduction.

Mouratoglou suggested the coaching methods have been used at "almost every match for decades".

While Mouratoglou was a vocal supporter of the ATP decision, Kyrgios – who pulled out of the Mallorca Championships with injury – hit back and slammed the proposed changes.

"Completely disagree. Loses one of the only unique traits that no other sport had," Kyrgios responded to Mouratoglou's post on Twitter.

"The player had to figure out things on his own. That was the beauty of it. What happens if a high-profile player versus a low-ranked player who doesn't have or [cannot] afford a coach?"

The trial commences on July 11 and will be evaluated at the end of the 2022 season, to assess the potential inclusion of off-court coaching in subsequent seasons.

Patrick Mouratoglou, the long-time coach of Serena Williams, has confirmed he will now work with Simona Halep.

Mouratoglou began coaching Williams in 2012 and has guided the former world number one to 10 grand slam titles and a further six major finals.

However, the last of those triumphs came in 2017, with Williams still one major success shy of matching Margaret Court's long-standing record of 24.

Williams reached the semi-final of last year's Australian Open, losing to Naomi Osaka, before subsequently going out in the last 16 at Roland Garros.

But the 40-year-old has not featured since retiring due to an injury in the first round to Aliaksandra Sasnovich at Wimbledon and is now ranked as the world number 246.

And it does not appear as though Williams will be making a return to the WTA Tour soon, with Mouratoglou explaining that, at least in the short term, he will be coaching former world number one Halep.

"Today, I am starting a new chapter in my coaching career: I am now the full-time coach of Simona Halep," Mouratoglou tweeted on Thursday.

"In the last eight months, I realised how much I missed coaching. It is the passion of my life, and I still feel like I have so much to give.

"Simona came to the Mouratoglou Academy before Indian Wells for a training block. I swung by at a few of her practices, watched her train.

"At the end of the week, she asked me if I was available to coach her. I have the highest respect for her but it was out of the question at the time.

"A few weeks later. I had a conversation with Serena, and the door opened for me, at least short term, to work with someone else.

"I will keep you updated on what's coming next soon."

World number 20 Halep endured an injury-hit 2020 and 2021, and split with long-time coach Darren Cahill in September of last year. The Romanian has won two grand slam titles, triumphing at Roland Garros in 2018 and Wimbledon the following season.

She has reached three other major finals and enjoyed a strong start to 2022, triumphing at the Melbourne Summer Set 1 prior to the Australian Open, in which she reached the round of 16.

Runs to the semi-finals at the Dubai Tennis Championship and Indian Wells Open have followed, though the 30-year-old will be hoping Mouratoglou can help her recapture the form she showed to end 2017 and 2018 on top of the WTA rankings.

Patrick Mouratoglou says Serena Williams' "heartbreaking" withdrawal from the US Open was the "only possible decision" for the American.

Williams on Wednesday revealed she will not play in her home grand slam due to injury.

The 23-time major champion has not played on the WTA Tour since suffering a torn hamstring at Wimbledon in June and she has not fully recovered.

Mouratoglou, Williams' coach, stated that pulling out of the final slam of the year was the only option for the six-time US Open singles champion.

He tweeted: "Since she had to pull out from Wimbledon, @serenawilliams has been fully committed to her recovery and we've done everything we could so that she could compete at the @usopen. But her body isn't ready. It is heartbreaking, but this is the only possible decision."

Williams posted on Instagram earlier in the day: "After careful consideration and following the advice of my doctors and medical team, I have decided to withdraw from the US Open to allow my body to heal completely from a torn hamstring,

"New York is one of the most exciting cities in the world and one of my favourite places to play – I'll miss seeing the fans but will be cheering everyone on from afar. Thank you for your continued support and love. I’ll see you soon."

Williams, who turns 40 next month, is the latest high-profile withdrawal from the tournament after Roger Federer and Rafael Nadal pulled out through injury

It will be the first time since 1997 that the US Open will be played without Williams, Federer or Nadal.