A bit of question for our Japanese friends on here. My impression is that it's a social etiquette in Japan to address people in their surnames, even among friends. But I'm presuming things have changed a little now especially among the younger generations? I'm asking because we're all calling Kei by his first name (obviously).
Actually I and my Japanese friends call him Nishikori-kun among ourselves. It's only when I speak or write in English I call him Kei. I do so because those ways sound more natural in either languages.
Calling someone with his/her surname is not formal at all in Japan if you add "kun" or "chan" or without any suffix. You are more likely to hear people calling out "Nishikori!" when cheering for him during a match here.
Maybe some people in Japan call him Kei or Kei-kun, too, and that sounds alright too IMO.
I saw on TV his mum talking with him on the phone and she was calling him "Kei-chan," which I think is the normal way for an elder family member to call him.
And let me elaboarate on what Kinokino said about his surname. It's not that the Japanese people find it difficult to pronounce the name phonetically. It's more to do with the Chinese characters used in the name.
The surname 錦織 is pronounced "Nishiki
ori" in many cases, and it's quite unusual to pronounce it "Nishikori." I read there are also rare cases in wich the name is pronounced "Nishigori" or "Nishikori" with a longer vowel "o." It all depends on family which way it should be pronounced, and in Shimane, where Kei comes from, it is apparently common to pronounce the name "Nishikori."
In any case, it is not a very common name, but I think it's a beautiful name. As you can probably see those Chinese characters mean "brocade."