Swiss 23rd seed and 2016 champion Wawrinka led Djokovic 6-4, 7-5, 2-1 when the Serb quit, having received treatment on his troublesome left shoulder before the start of the third set.
Williams, a six-time US Open winner, romped to a 6-3, 6-4 victory over Croatian 22nd seed Petra Martic to set up a quarter-final with China's Wang Qiang, the conqueror of world number two Barty.
Tonga have been drawn with England, France, Argentina and the United States in Pool C, with only the top two teams to advance to the quarterfinals.
International Motoring Federation (FIA) race director Michael Masi said an investigation into the crash on Saturday that killed Formula Two driver Anthoine Hubert had already begun.
Harry Maguire said United must take their opportunities in front of goal if they are to keep up with their top four rivals.
Several shops were looted and set alight in Malvern in the Johannesburg CBD area on 1 September 2019. The looting and violence spread overnight to Denver and other areas.
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Ntseki started out a school teacher before the coaching bug hit him and he followed his passion.
The 42-year-old man appeared before the Wynberg Magistrates Court on Monday morning where he confessed to sexually assualting and murdering her.
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Aggrieved truck drivers have been blockading major roads in protest against a lack of job opportunities.
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According to reports, doctors at the Los Angeles hospital where he is being treated have optimism he can recover from his injuries, as he didn't suffer any spinal cord damage during the crash.
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