Torn meniscus was removed in an arthroscopic procedure.
The German earned a breakthrough win in denying Williams a record-tying one.
They both scored relatively easy wins as they look to defend their Melbourne titles.
As he retires, Hewitt's legacy as a fighter will carry on after his pro career ends.
The incident follows published reports of widespread corruption in the sport.
Jo-Wilfried Tsonga probably gained a few new fans for the display of kindness.
"It's not true," world No. 1 says after latest Australian Open win.
Venus Williams, Simona Halep also sent home early in similarly shocking upsets.
Djokovic's age and his dominance of men's tennis put him on track to be the best ever.
A blockbuster report overtakes the Australian Open.