Defending champion Federer said:
“I’ve just been trying to manage the situation really. Back spasms happen and sometimes you can contain them with massage and painkillers,”
“I don’t want to drive myself crazy with more painkillers and I need to do what’s right to get it better. It is only the second time I have pulled out from a tournament and I have never pulled out during a match. It’s a sad moment for me, the tournament and the fans, but health comes first.”
However, the Swiss is hoping to be fit for the upcoming Australian Open.
Federer beat Grega Zemlja and Andreas Seppi on his way to the semis.
In the other semifinal, Gael Monfils surprisingly beat world no.2 Rafael Nadal 6-3 6-4.