The American, who looked unwell throughout the match, putting iced towels around her neck and against her forehead at changeovers, fell a set and a break down as the 23rd-seeded Bacsinszky peppered the court with backhand winners.
               
But Williams, who won Roland Garros in 2002 and 2013, dug deep to break back and Bacsinszky cracked, losing 10 games in a row.
               
Williams closed it out when Bacsinszky sent a backhand long and the world number one will now face Czech 13th seed Lucie Safarova on Saturday.
               
"I have tried everything. I thought if I lose I must fight. I have tried and tried, I don't know where I found the energy," Williams told a courtside interviewer in French before a coughing fit ended the conversation.

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