There have been numerous high-profile flashpoints as a consequence of tennis players receiving instructions from their coaches during matches over the years.
In-match coaching used to entail a greater risk of code violations, with some of the leading lights of the sport among those punished for taking advice while playing.
There are fewer restrictions on coaching, though, under the changes to the rules approved in mid-2022, which were met with a mixed reaction from players and coaches.
What are the current rules and how much are players allowed to be coached during matches? Here's what you need to know for the upcoming US Open and beyond.
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Off-court coaching: What were the rules before?
Communications between players and coaches were generally banned, although regulations around coaching used to vary substantially between men's overseeing body the ATP, women's equivalent the WTA and different types of tournaments and levels of the sport – and they still do, to a lesser extent.
In contrast to male players, for example, female players were allowed to request coaching on-court once a set as long as it wasn't a grand slam tournament. In-match coaching rules in the women's game have been increasingly eased since 2008.
Governing body the ITF also permits players to freely receive coaching at all times during Davis Cup and Federation Cup matches.
No coaching is allowed when players take bathroom breaks, which is part of the reason why a tournament official accompanies players when they request one. But they can speak to their coaches if their match is suspended, which might partly explain why the momentum of matches sometimes changes when there is a long mid-match delay.
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How have the rules around in-match tennis coaching changed?
Although conversations during matches are still not allowed, coaching with restrictions is now permitted as part of rules initially brought in at the 2022 US Open.
The Australian Open also implemented the trial in January 2023, with the French Open and Wimbledon following suit.
Coaches can offer brief verbal instructions and provide guidance through gestures but they still can't have a conversation with their players during breaks in play, when there is a brief match delay or when competitors take a bathroom break.
What are the penalties and punishments for off-court coaching?
Players are initially given a warning when an umpire considers them to have communicated with a coach in a way or at a time that is not allowed.
A second instance of the violation results in a point penalty — losing a point — and a third results in an entire game being forfeited.
One of the most explosive examples of a violation came in the US Open final in 2018, when Serena Williams was furious with umpire Carlos Ramos after he penalised her for receiving guidance from her coach, Patrick Mouratoglou, who appeared to be subtly gesturing.
It was no surprise to see Mouratoglou publicly supporting the decision to change the rules. "Congratulations to the ATP for 'legalising' a practice that has been going on at almost every match for decades," Williams' coach for 10 years wrote on Twitter. "No more hypocrisy."
Another player who has been coached by Mouratoglou, Stefanos Tsitsipas, has repeatedly landed violations for coaching, including one when his father, Apostolos, was also punished during his son's defeat to Daniil Medvedev in the 2022 Australian Open semifinals.
What have tennis players said about the changes to coaching rules?
There have long been conflicting arguments around the issue. Supporters of the changes believe that tennis should be no different to many other sports in allowing coaches to direct players during matches, while others feel that an individual sport should not encourage outside influence while the action is taking place.
Tennis legend Roger Federer said he was "not all for it". "I find it kind of cool that, in tennis, you're sort of on your own out there," the 20-time grand slam title winner said in 2017. "Not everybody has the same amount of resources for coaching, as well. So I'm not sure if it's that beneficial."
Rafael Nadal's coach, Carlos Moya, is not in favour. "What makes tennis a special sport is that it is the only sport where you are alone against another, without help from anyone," Moya told Eurosport when the rule changes were mooted.
"In the rest of sports you do have that contact in some way, be it with the presence of your coach, through the radio, with the caddy, whatever.
"That solo battle that the tennis player has — placing the pieces of the puzzle at 180 beats and 20 seconds between points, choosing the correct tactic in each play — is part of the quality of each player.
"For me, it is essential that the player thinks for himself. The coach’s job is already done. It’s like an exam: the teacher can help you, but once the exam starts you’re on your own. The coach’s job has to be done before the match. The player must have controlled all the variants because then things will happen that surely weren’t in the script."
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Twelve-time grand slam tournament champion Billie Jean King has said she doubts the impact changing the rules can have.
"People say in tennis you have to think for yourself, but I've told players countless times what to do and they can either do it or they can't. It's up to them anyway."
2021 US Open winner Medvedev was similarly ambiguous. "During the match, I don’t really see how a coach can help like in some other sports, where tactics are so important," he said, speaking before a match against Taylor Fritz.
"I guess it can be one match out of five where suddenly a coach is going to be, like, 'change your position on return' or 'go more to his backhand', if he sees it from the outside. So I think it can be a game-changer sometimes.
"At the same time, in many matches... let’s say I play Fritz and it’s our first encounter. I know how he plays and he knows how I play. I don’t think on-court coaching tomorrow is going to make a huge difference. But in general, I’m okay with it – I was never against it."
Denis Shapovalov warned that allowing coaching could "change the tradition" and Fritz said he "really hated" the idea, adding that it is "not something that should be a part of our sport."
Medevedev's coach, Gilles Cervara, said the rule change would end the "hypocrisy" and "lack of consistency" in applying penalties during matches, adding to Tennis Majors: "Tennis is such a cerebral sport that, for some players, a talking coach is more annoying than anything else."
Tsitsipas said he was "glad" about the change. "I see no reason to have a coach with you if they can't share some of their view and knowledge with you when you’re competing," said the Greek.
"I feel like it’s something very natural in our sport. It’s happening with almost every single player. The fact that it’s legalised now is going to make tennis a bit more peaceful and make players concentrate more on the game, less on a different kind of nonsense.”
Even after the trial of the rule change began, there were altercations around mid-match coaching. "These people keep talking, point in, point out," number 23 seed Nick Kyrgios shouted at his box at the US Open during his straight sets first-round singles win over his usual doubles partner, Thanasi Kokkinakis. "But he’s allowed to do it. It’s legal now – it’s legal."
The characteristically outspoken Kyrgios had already disagreed with Mouratoglou's point on Twitter. "The player [previously] had to figure out things on his own," he told him.
"That was the beauty of it. What happens if it's a high-profile player versus a low-ranked player, who doesn’t have or can't afford a coach?”
2022 Wimbledon champion Elena Rybakina's coach, Stefano Vukov, was criticised for being too aggressive when he took advantage of the new rules during his player's run to the 2023 Australian Open final.
During Rybakina's defeat to Aryna Sabalenka in Melbourne, former world number one Pam Shriver tweeted: "As I watch Rybakina try to win her second major in seven months, I hope she finds a coach who speaks and treats her with respect at ALL times and does not ever accept anything less."
Rybakina responded: "He is a passionate coach with a lot of knowledge about tennis. Unlike people that are making these comments, he has great knowledge about me as a person and as an athlete.
"Those who know me well will know that I would never accept a coach who didn’t respect me and all our hard work. I may be quiet on court and in general, but inside me is a competitive athlete who wants to achieve great things – and Stefano has helped me greatly in this way. So please disregard any fake news to the contrary."
While explaining himself, Vukov mentioned one of the new challenges presented to coaches who try to advise players during matches.
"This is part of our sport, it’s normal," he said. "There are 10,000 people out there; to get the attention of the player is definitely not easy and people don’t understand that. I have to scream out something if she’s off track.
"People can interpret that how they want but at the end of the day, we’re just doing our job. Coaching is now allowed and she’s using it in the best possible way."
For American prospect Ben Shelton's coach, Dean Goldfine, the "big grey area" is the question of what constitutes a conversation between player and coach.
"It’s a little ridiculous, just from that standpoint,” said Goldfine, telling the New York Times that being allowed to receive coaching during matches had been "huge" for Shelton.