MELBOURNE, Australia — Dominant in the final Grand Slam tournament of 2017, the American women had their fortunes take a dramatic turn for the worse on the opening day of the first major tournament of 2018.

Venus Williams, whose terse news conferences are becoming a trademark, summed up the abrupt shift in momentum succinctly.

“Last year is last year,” she said. “You can’t live in the previous year. It’s impossible.”

Williams, an enduring champion, was one of four American women to reach the semifinals at the United States Open in September, but after Monday’s stumbles, the only one of those four women still in the singles tournament here is Madison Keys, who will play her first-round match on Tuesday.

The first to fall was Sloane Stephens, the big-surprise U.S. Open champion. Stephens, seeded 13th here, has yet to win another match since that feel-good breakthrough and was beaten on Monday in Margaret Court Arena by Zhang Shuai of China, 2-6, 7-6 (2), 6-2.