Roger Federer continues to defy the ageing process but even the great Swiss will know the window of opportunity to add to his Grand Slam haul is growing narrower every passing month.
Angelique Kerber, who won the Australian Open in 2016, had on-court medical help for her lower back injury and then retired in her match against Dayana Yastremska.
A Slovenian player suffers a coughing fit as smoke, poor air quality blight Aus Open qualifying event.
While Prajnesh Gunneswaran will face Germany's Yannick Hanfmann in the second-round, Sumit Nagal will open his campaign in the qualifiers against Egypt's Mohamed Safwat on Wednesday.
Slovenia's Dalila Jakupovic was leading Stefanie Vogele at Melbourne Park when she slumped to her knees at the back of the blue hardcourt suffering breathing difficulties because of the bushfire smoke.
Returning to the court after two years, Sania Mirza and Ukrainian partner Nadia Kichenok prevailed 2-6, 7-6 (3), 10-3 over Oksana Kalashnikova of Georgia and Japan's Miyu Kato in a contest lasting one hour and 41 minutes.
Novak Djokovic recorded a ninth consecutive hard-court win over world number one Rafael Nadal on Sunday as Serbia were crowned champions at the inaugural ATP Cup team event.
With the final locked at 1-1 after the singles were shared, Novak Djokovic partnered veteran Victor Troicki to beat Spain's Pablo Carreno Busta and Feliciano Lopez 6-3, 6-4 and win the title.
Sania Mirza and her partner Nadiia Kichenok have been drawn to meet Oksana Kalashnikova of Georgia and Miyu Kato of Japan in their opening round at the Hobart International.
The 2017 and 2019 winner broke again for 6-5 and then served it out to clinch victory in her first meeting with the eighth-seeded American in a little over two hours.
The 38-year-old Williams now has the opportunity to sweep both titles at the Auckland Tennis Centre, having also advanced to the doubles final with good friend Caroline Wozniacki.
Karolina Pliskova and Naomi Osaka served 29 aces between them and were often locked in fierce baseline rallies before the second-seeded Czech won 6-7(10), 7-6(3), 6-2 in two hours and 48 minutes.
Ekaterina Alexandrova, playing in only her second WTA title clash, breezed through the first set and staved off a potential comeback from Elena Rybakina in the next to become the first tour champion of 2020.
Bianca Andreescu, who suffered a knee injury at the WTA Finals in Shenzen, is undergoing a rehabilitation programme now, and had earlier opted out of the Auckland Classic.
While Spain's Rafael Nadal recovered to win 4-6, 7-5, 6-1 against Australia's Alex de Minaur, Serbia's Novak Djokovic picked apart Russia's Daniil Medvedev at the inaugural ATP Cup by 6-1, 5-7, 6-4.
It was Naomi Osaka's third consecutive three-setter in the tournament and the former world number one eventually prevailed in two hours to reach the semi-finals for the second year in a row at the Pat Rafter Arena.
Rafael Nadal recovered from an upset loss against Belgium's David Goffin to win the deciding doubles match and send Spain into the semi-final of the ATP Cup teams event.
Keys, a 2017 U.S. Open finalist, reached the semifinals in Brisbane for the first time in four attempts.
The victory, set up earlier by Serbia's Dusan Lajovic's upset win over Canadian teenager Felix Auger-Aliassime 6-4 6-2, means Serbia will play Russia in the semi-final. That clash will see Djokovic face world number five Daniil Medvedev
Siegemund, who beat American teenager Coco Gauff in the second round, stretched Williams with her variety and placement but couldn't match Williams' power from the baseline.