In pictures: Day 8 highlights of Australian Open
World no.1 Simona Halep suffered an embarrassing defeat at the hands of Williams while Novak Djokovic continues his bid for a hat-trick of Grand Slam titles after defeating Daniil Medvedev.
Novak Djokovic struck back for the old guard at the Australian Open, soaking up the pressure from a willing Daniil Medvedev before crushing the young Russian 6-4 6-7(5) 6-2 6-3 to reach his 10th quarter-final at Melbourne Park.
The world number one will continue his bid for a hat-trick of Grand Slam titles and a record seventh at Melbourne Park against eighth seed Kei Nishikori.
Japan's Kei Nishikori
Exhausted Kei Nishikori battled from two sets down to outlast Spain's Pablo Carreno Busta 6-7(8) 4-6 7-6(4) 6-4 7-6(8) in an epic encounter.
Dragged into a five-set dogfight for the third time in four rounds, Nishikori struggled to deal with the physical and mental strain early on as Carreno Busta took a two-set lead but the eighth seed clawed his way back to level the contest.
Alexander Zverev only managed to beat his racket as the highly-regarded fourth seed was dumped out of the Australian Open by big-serving Canadian Milos Raonic in a surprisingly one-sided 6-1 6-1 7-6(5) fourth round triumph.
Zverev has emerged as one of the brightest stars in men's tennis after victories over Roger Federer and Novak Djokovic en route to an ATP Finals triumph in November, but the 21-year-old German remains unable to find his best form at Grand Slams.
Zverev started well against 16th seed Raonic with a break in the first game, but he was then broken six times in the first two sets before the Canadian converted his fourth match point in a tight third set to seal the contest in less than two hours.
Injury-prone Raonic, who climbed to a career-high ranking of third in 2016 when he reached the Wimbledon final, believes he was rediscovering his best form.
Simona Halep and Serena Williams
Serena Williams on Monday defeated Romanian tennis star and current world no 1 Simona Halep 6-1, 4-6, 6-4 to cruise into the quarter-final of the Australian Open.
Williams clinched the first set in the blink of an eye. It was a spectacular set from the 16th seed and she took just 20 minutes to make the world No1 look very ordinary indeed.
Naomi Osaka and Anastasija Sevastova
US Open champion Naomi Osaka completed another comeback victory to overhaul 13th seed Anastasija Sevastova 4-6 6-3 6-4 and reach the quarter-finals of the Australian Open.
The Japanese fourth seed, who had to fight back from a set down against Hsieh Su-Wei in her previous match, was far from her best as she struggled to hold serve and berated herself regularly during the sun-bathed clash at Rod Laver Arena.
Karolina Pliskova thrashed double Grand Slam champion Garbine Muguruza 6-3 6-1 on Monday to reach the Australian Open quarter-finals for the third year in succession.
The Czech seventh seed broke former French Open and Wimbledon champion Muguruza five times in the one-sided clash in the afternoon sunshine at Margaret Court Arena.
Needing only 24 minutes to win the second set, Pliskova earned two match points on Muguruza's serve and sealed the win when the out-of-sorts Spaniard smashed a backhand long.