A roar came when Williams authored a superb backhand yet that was far from the end of the tale. Williams, surprisingly, was broken to love to trail 5-4. Perhaps at that instant some flashed back to 2012 at Roland-Garros, the lone occasion Williams has lost at the first hurdle of a major in her glittering four-decade career.
Yet those thoughts likely quickly evaporated as Williams emphatically broke back for 5-5. Ahn commendably forced a tie-break but Williams did not make a single unforced error after delivering 28 in the first 12 games.
Ahn missed her opportunity and somewhat predictably, the 74-minute first set gave way to a 27-minute second.
It will be another familiar opponent for Williams next in Tsvetana Pironkova as she continues her chase for Margaret Court’s Grand Slam record. The comeback Bulgarian — a mum, too — stretched Williams to three sets in an absorbing quarter-final in New York.
“She's playing well, but I am too,” said Williams. “I'm ready to play her. She'll be ready to play me. It will be a long match, she will get a lot of balls back. I’ll be ready.”