Following several deuces, Cornet struck a powerful forehand down the line return which Williams returned with a netted backhand crosscourt to reach breakpoint.
The American hit a crosscourt crosscourt forehand wide to drop serve as the set as leveled at 3 all.
Serving at deuce, Cornet struck a crosscourt backhand wide to face breakpoint.
The Frenchwoman then hit a crosscourt backhand long to lose serve to fall behind 3-4.
However in her following service game at deuce, Williams hit two backhand unforced errors to drop her service game to level the set again at 4 all.
With Cornet serving, Williams struck several powerful groundstrokes to reach 0-40 before the Frenchwoman hit a down the line forehand long to drop serve and trail 4-5.
With Williams serving, Cornet struck a crosscourt backhand lob winner which in response Williams gave Cornet a stare across the net as she fell behind 0-15.
The American then hit two crosscourt forehands wide to trail 0-40.
Williams netted a down the line forehand to lose serve as the set was evened once again this time at 5 all.
The final game of the match lasted more than nine minutes as Cornet struggled with her serve and several powerful returns from Williams.
The American, who struck no aces for the day, appeared despite moments of extreme power to be winded after several points.
Late in the set, Williamsí facial expression also appeared negative after several points similar to the expression the American had in another loss to Cornet earlier this year at Wimbledon.
Cornetís final service game went to a fifth deuce as the Frenchwoman struck her third double-fault of the game.
Williams then struck an inside out crosscourt backhand return winner to set up breakpoint.
In the final point of the match, Cornet hit a drop shot lob combination which Williams ended the point with an overhead winner to lead 6-5.
On the changeover, Williams sat with an ice towel wrapped around her shoulders before the trainer came oncourt to give her a thermometer.
The chair umpire then announced the American was to retire from the match with an illness.
Williams left the court walking with her arm wrapped around a WTA Tour official as it was later announced she suffered a viral illness.