Back in December, Craig Tiley said that it was "very likely" that Williams would be back to defend her Australian Open title, saying that "There is no question that she'll be ready" in the view of Tennis Australia.
"After competing in Abu Dhabi I realised that although I am super close, I'm not where I personally want to be", Williams added. "I want to do far better than that and to do so, I will need a little more time".
On medical advice, he also withdrew from the Qatar Open and now faces a final fitness test for the Australian Open.
Nishikori, now ranked 22 in the world, also plans to compete at the Challenger event in Dallas, Texas, which begins on January 29.
"I've been in constant contact with Serena and her team and know this is why she has pushed it and pushed it until the 11th hour to make her final decision", he said.
Three-time Abu Dhabi victor Djokovic joins a six-strong field that also features Dominic Thiem, Kevin Anderson, Pablo Carreno Busta, Roberto Bautista Agut and Andrey Rublev. "I'll be flying home shortly to assess all the options but I appreciate all the messages of support and I hope to be back playing soon".
"Sorry for the long post but I wanted to keep everyone in the loop and get this off my chest as it's really hurting".
Former world number one Djokovic hasn't played since a right elbow issue forced him to quit Wimbledon in the quarterfinals in July. He plans to play in two exhibition tournaments in Melbourne next week before deciding.
The 28-year-old, who reached a career high world number four in 2015, said in November he had chosen rehabilitation over surgery to fix the damage and was targeting a return to action in Australia.
With Serena Williams out, the women's singles title at Melbourne appears to be wide open.
"The team carefully evaluated Kei coming back in Melbourne".
Tiley said he was in daily touch with Williams' management.
'I was hitting the ball well from the first practice but the thing is that your muscles, and bone structures and ligaments and joints, ' said Djokovic, who has slipped down to 12 in the world rankings following his enforced break.
AT&T will release true 5G service by late 2018
AT&T was the subject of some jibes past year when it announced its " 5G Evolution " markets that employ LTE Advanced features. We also still have no idea how fast AT&T's 5G network will be when it does launch in each of those 12 cities.