During a match at Wimbledon in June, Williams suffered an injury to her right leg and has not competed since.
"I am greatly disappointed," the tennis star tweeted, "as it has been an honor and highlight of my career each time I have represented Switzerland."
Federer faces Poland's Hubert Hurkacz, the No. 14 seed, while Djokovic takes on unseeded Hungarian Marton Fucsovics.
Coco Gauff plays former Wimbledon champion Angelique Kerber, while Roger Federer takes on No. 23 seed Lorenzo Sonego at Wimbledon.
The 23-time Grand Slam champion was forced to withdraw from her first-round match with an ankle injury.
The 20-time Grand Slam champion takes on France's Adrian Mannarino as he begins his search for a ninth Wimbledon crown.
World No. 1 Novak Djokovic looks highly likely to equal the all-time record of 20 Grand Slam singles titles held by Roger Federer and Rafael Nadal.
"Well, I don't know the boy, but he was in my ear the entire match," said Djokovic, who won his 19th Grand Slam title with the win.
Rafael Nadal chases a record 21st Grand Slam singles titles, while Serena Williams aims to equal Margaret Court's record of 24 Grand Slam crowns.
From Roger Federer to NBA and MLB athletes, to Cristiano Ronaldo and Lionel Messi, a host of stars have donated to the fight against the pandemic.
The winners of the last five Australian Opens meet in the semifinals at Melbourne Park for the first time since 2016.
"I found it a bit tough," He added. "It's not like I'm known to throw around words and whatever. It's not like the whole stadium heard it either, but no problem, I'll accept it," Federer said of the violation.
Federer takes on Tennys Sandgren of the U.S., while Djokovic faces No. 32 seed Milos Raonic of Canada.
The world No. 1 takes on Spain's Pablo Carreno Busta, while Medvedev plays Australia's Alexei Popyrin.
Williams faces China's Wang Qiang, while Federer takes on Australia's John Millman and Coco Gauff plays Naomi Osaka.
Williams faces Slovenia's Tamara Zidansek, while Federer takes on Serbia's Filip Krajinovic and Djokovic plays Japan's wild card Tatsuma Ito.
The first Grand Slam of the season begins in Melbourne on Monday, with some of the tournament favorites all in action.
The 108th edition of the Australian Open gets underway in Melbourne on Monday, with the women's and men's final scheduled for the first weekend of February.
Novak Djokovic and Serena Williams are the bookmakers' favorite to add an eighth Australian Open title to their collections this year.
The first Grand Slam tournament of 2020 gets underway on January 20 in Melbourne with Novak Djokovic and Naomi Osaka looking to defend their titles.
Roger Federer, Novak Djokovic, Rafael Nadal and Serena Williams are among the players in action in the fundraiser for the victims of the bushfires.
Environmental activists have called for Federer to cut ties with a company that has invested millions in fossil fuel.
The Spaniard is odds-on favorite to win a 19th Grand Slam title after Novak Djokovic and Roger Federer were both knocked out.
Federer chases a first semifinal at Flushing Meadows since 2015, while Williams looks to take another step towards a 24th Grand Slam title.
Federer takes on Britain's Dan Evans, while Djokovic faces American qualifier Denis Kudla on Friday evening.
Coco Gauff plays Hungary's Timea Babos, while Rafael Nadal takes on Australia's Thanasi Kokkinakis on Thursday.
The world No. 1 and the world No. 3 are both in action on Wednesday, as are the Williams sisters.
Rafael Nadal and Coco Gauff are both in action on Tuesday as the first round of the U.S. Open is completed.
The world number one takes on the number 23 seed as he looks for a second consecutive Wimbledon final.
Federer and Nadal will renew their rivalry on Centre Court on Friday, 11 years after they last met in an epic final.