Victoria Azarenka reached the US Open final in 2012 and 2013 where she was beaten by Serena Williams on both occasions.
Rafael Nadal dismissed Karen Khachanov in straight sets, Victoria Azarenka got the better of British hope Heather Watson in three sets.
With the 23-time Major winner preparing for the birth of her first child and the Russian out with an injury, the race to the Venus Rosewater Dish is wide open.
Former world No 1 will return to competition seven months after giving birth to son Leo.
Serena Williams will again be the cynosure of the women's singles competition in the Australian Open. She will face Switzerland's Belinda Bencic in a difficult first-round clash on Monday.
Victoria Azarenka had to retire from her opening round tie against Karin Knapp at Roland Garros with the knee problem
Nick Kyrgios defeated 2014 Madrid Open finalist Stan Wawrinka while Victoria Azarenka withdrew from the tournament due to a back injury.
Victoria Azarenka and Petra Kvitova won in straight sets to enter the Madrid Open third round
With the world's dominant player Serena Williams missing for so much of the season, 16 different women have already lifted WTA titles this year with Sloane Stephens, Victoria Azarenka and Angelique Kerber the only multiple winners.
Victoria Azarenka beat two-time Grand Slam champion Svetlana Kuznetsova of Russia 6-3, 6-2 for her third Miami Open title.
Victoria Azarenka jumped seven places to eighth spot in world rankings.
Victoria Azarenka blasted five aces while Serena Williams was undone by 33 unforced errors and five double faults in the Indian Wells final
Victoria Azarenka routed Slovakia's Magdalena Rybarikova 6-0, 6-0 in the Indian Wells quarter-finals.
Victoria Azarenka, who was stunned by Germany's Angelique Kerber in Australian Open quarters, blamed herself for not taking her chances in a match in which she had many.
Angelique Kerber defeated two-time champion Victoria Azarenka 6-3, 7-5 in the Australian Open quarter-finals.
Two-time champion Victoria Azarenka was too strong for the Czech, winning 6-2, 6-4 on Rod Laver Arena to set up a last-eight clash with seventh seed Angelique Kerber at the Australian Open.
The former world number one was in ominous touch as she thrashed the teenage qualifier 6-1, 6-1 in 56 minutes on Rod Laver Arena to make it eighth consecutive last-16 appearances at Melbourne Park.
Victoria Azarenka in fine touch as she hammered Montenegro's Danka Kovinic 6-1, 6-2 on Margaret Court Arena to progress to the third round of the Australian Open.