There is such a thing as being brutally honest. But Serena Williams' coach thinks Rafael Nadal's honesty could be self-destructive.

Tennis.com reported that Patrick Mouratoglou, coach and rumored boyfriend of Williams, the No. 1-ranked women's player and 18-time Grand Slam champion, told L'Equipe he was stunned to hear that Nadal, the No. 5 men's player and owner of 14 Grand Slam titles, "claimed he was 'weak' on court."

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After losing to countryman Fernando Verdasco in the third round of the Miami Open last month, Nadal gave a frank assessment of the state of his game. He said physically he felt fine, but his lack of preparation had an effect on his confidence.

"It's not the question of tennis, [It's] the question of being relaxed enough to play well on court," the 28-year-old, according to atptour.com. "Today, my game in general improved since a month and a half [ago]. But at the same time, [I'm] still playing with too many nerves for a lot of moments, in important moments."

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The comment blew Mouratoglou away.

"There are two ways to approach a problem-deny it or recognize it, which is what he did. But this time he went further, and he did it publicly, and it's something that I had never seen before," Mouratoglou said. "Serena would never have done that. Nadal's team said over the years that Rafa is not good, and does not play well, his opponents are 10 times stronger. So for me what is happening is the continuation of this process. It could be a strategy, or a way to exorcise the pressure, but I could be wrong and maybe it's just a part of their personality to be so transparent."

Nadal's coach and uncle, Toni Nadal, also expressed concern for Rafa as he looks to defend his French Open title, saying the player is as vulnerable as he's ever been as Roland Garros approaches. Nadal has won nine of the past French Opens, including five in a row.

Even if the transparency is by design, it's a risky move, to say the least, according to Mouratoglou.

"Rafael Nadal is one of the mentally stronger players, and to say that now he is weak in that aspect, it is surprising," Mouratoglou said. "I do not mean that the problem isn't there, but for some years there wasn't a player who thought he could beat him on clay. They all went on the court already defeated. Now things are sure to change."