^ He's been wearing it since the match against Benny. Wore it in RG too.
Some say it's to keep the back warm in these conditions...
I didn't see him wear it in RG during the matches - onlu at the practice but at Wimb he put it after he asked the trainer help against Malisse.
I confirm it's a thermo t-shirt to have more hot at the back when it's cold and windy.
It couldn't be easy play with it because you can faint by the hotness like yesterday with the closed roof against Djokovic and at half match he removed it.
I've just wondered why he put the black one and not a white
Maybe he had only black for practicing.