U.S. Open champion Sloane Stephens was first to go, followed quickly by last year's Australian Open runner-up, Venus Williams.
Only last September Americans filled all four semi-final slots at their own end-of-year major, so the crash on Monday was loud and painful.
The volatile 17th seed was on his best behaviour, apart from one first-set rant about a heckler in the crowd, letting his racket do the talking as he treated his fans in the Hisense Arena to an impressive opening salvo.
"I'm so happy to play on this court and play a great player". "I just have to give her credit for that". I like her very much and she's always nice to me. "When I came back here to warm up I was feeling so excited. It means a lot".
Bencic broke Williams serve in final game of the match, winning the last four points. She returned on a roll, winning the next six points to clinch the first set, and then dominated the second.
Zhang raced out to a 3-0 lead in the second set, breaking the American early thanks to a variety of errors from Stephens. Venus praised her opponent for playing "above and beyond" but was otherwise tight-lipped and sullen during her post-match media duties.
"I did everything that I could to be ready for Sydney last week and for here", said Stephens when asked about her knee problem. And, when asked to comment on a recent Vogue article in which Serena outlined her serious medical difficulties giving birth a year ago, she would only say, "Serena is definitely a tough individual".
Zhang Shuai was her latest conqueror with a 2-6 7-6 (7-2) 6-2 victory, but Stephens was not too downhearted and is confident her form will return.
"I love her", Stephens said. "I tried my best". The 10th-seeded American lost 7-6 (7-4) 6-2 to 57th-ranked Timea Babos.
The other American casualties suffered variously.
In-form 12th seed Julia Goerges also advanced and will take on Alize Cornet in the second.
The Latvian recovered from 4-1 down in the second to win 6-1 6-4 to set up a tie with Duan Yingying.
Belgium's Kirsten Flipkens had to fight hard to get past Alison Riske 2-6, 7-6 (6), 6-3.
The 18-year-old Canadian, whose rise to 50th in the ATP rankings in the past 12 months has been meteoric, won 6-1 6-3 7-6 (5) against the 19-year-old he also beat on his way to claiming the Wimbledon junior title in 2016. The Chinese got off to an early start in the breaker and was able to close it out, winning 7-2. Since beating Madison Keys in the U.S. Open final, Stephens has lost eight matches.