Six-time Grand Slam doubles champion Joe Salisbury is taking a break from tennis and is not planning to return before April due to anxiety.
Salisbury, who played in the doubles showpiece alongside Neal Skupski at the ATP Finals in Turin at the weekend, losing out to Henry Patten and Harri Heliovaara, said the discomfort has intensified over the course of the season.
The 33-year-old, who has won a hat-trick of US Open titles, will now be looking to take time away from the court for the foreseeable future.
In an interview with BBC Sport, he said: "I've had heart palpitations, a feeling of your heart beating all around your body.
"You feel like your body is almost a bit shaky, almost like your whole body is vibrating a little bit.
"It's been tough to deal with at times and it's made me not want to be playing and competing at times throughout the year."
"It's definitely a lot in the stomach - you feel sick to your stomach.
"I've been struggling to sleep and then, because of the feeling in the stomach, struggling to eat well, eat enough.
"It's almost like a feeling of dread - that something bad is going to happen."
The current world No 10 continued: "I wouldn't say it affected my tennis that much.
"We've played well, especially the past six months. I feel like I've dealt with it well and managed to get myself into a good enough state on court to perform well in most of the matches that I have played.
"But I think it's taken an extra toll emotionally and mentally to do that. It's meant that it's just not been enjoyable being at a lot of the tournaments that I have played."
Salisbury, who is only the second British doubles player to reach world No 1 after Jamie Murray in 2016, admitted he is happy to discuss his issues publicly, adding: "I'm sure it's something which a lot of other people are having to deal with and I don't think it would have any impact on me, as if anything over the past year it's made me mentally stronger than before."
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