"This is, like, a new year," Williams, who was seeking to become the oldest grand slam winner in tennis history, told reporters. "You can't live in the previous year. It's impossible.

"I don't think I played a bad match. She just played above and beyond. I just have to give her credit for that."

With Serena skipping the event because the record 23-time grand slam winner isn't ready to return to tournament play four months after giving birth to a first child, it means no Williams sister is in the second round of a major for the first time since 1997.

US woe

The fourth American semifinalist at Flushing Meadows, Madison Keys, is still in contention -- she won't contest her opener against China's Wang Qiang until Tuesday.

And men's eighth seed Jack Sock, a semifinalist at the year-end Tour Finals in London in November, tumbled at the hands of Japan's Yuichi Sugita 6-1 7-6 (7-4) 5-7 6-3.

Of the 15 Americans on the schedule Monday, only three -- Ryan Harrison, qualifier Mackenzie McDonald and Nicole Gibbs -- prevailed.

Federer's parents watch Bencic beat Venus

Bencic encountered a Williams sister in the first round for a second consecutive Australian Open -- she lost to Serena 12 months ago -- but this time around the former prodigy was in much better form.

Having climbed to seventh in the rankings in 2016, back and wrist injuries heavily contributed to the Swiss dropping to 317th last season.

Bencic, however, finished 2017 by going 28-3 at all levels and combined with Roger Federer to help Switzerland win this month's Hopman Cup. Federer's parents were in the 20-year-old's player box at Rod Laver Arena.

The Swiss took the final four games of the first set, then rallied from a break down in the second to register a first win over Williams in five attempts.

"It was amazing, this match," Bencic told reporters.

"I think the level was great. Also she beat me four times before, so I was really happy, took my chance this time.

"Obviously just happy to be back playing. It was a very, very long time when I was out. I couldn't wait to come back, so it means a lot."

Given Bencic's form, don't discount a long stay in Melbourne. With Serena out of action -- just as she was for the final three majors of 2017 -- the women's draw is fairly wide open as Vandeweghe's conqueror, Timea Babos, acknowledged.

"In women's tennis, I think we see it -- anything can happen when there is no Serena Williams," said the Hungarian, who triumphed 7-6 (7-4) 6-2.

Vandeweghe received a code violation from chair umpire Fergus Murphy, as she wasn't ready to take to the court because she was eating a banana. Vandeweghe said she had to wait for the bananas -- a good source of potassium -- to be brought courtside, hence the delay.

Eight straight losses for Stephens

Stephens was ruled out of this month's Brisbane International with a knee injury and lost in straight sets to Camila Giorgi in the first round in Sydney, hardly the best preparation for Melbourne.

Not serving out the match at 5-4 in the second set Monday was the obvious turning point and the 13th seed went out to China's Zhang Shuai -- somewhat of a fan favorite at the Australian Open -- 2-6 7-6 (7-2) 6-2 to suffer an eighth straight defeat.

"I did everything that I could to be ready for Sydney last week and for here," Stephens told reporters. "It's just unfortunate, but it's definitely only the beginning of the season, first two tournaments. Not going to get too down."

Zhang was close to quitting tennis following an 0-for-14 skid at grand slams. Her fortunes changed, though, when she upset Simona Halep in the first round in Melbourne two years ago and last week she narrowly missed out on a seeding for Melbourne, coming in ranked 34th.

Rafael Nadal progressed to the second round in 94 minutes, downing 37-year-old Victor Estrella Burgos 6-1 6-1 6-1. It was a flawless start from the world No. 1, who is playing the Australian Open for the first time without competing at a warmup tournament. Blame his knees.

Nadal drew as much attention for his attire, going back to a sleeveless look for the first time in nearly a decade.

The man Nadal lost to in last year's gripping men's finale, Federer, begins his quest for a 20th major Tuesday against Slovenia's Aljaz Bedene.

Six-time Australian Open champion Novak Djokovic, meanwhile, plays his first official match since Wimbledon against Donald Young.

If Young topples Djokovic, it would no doubt bring some joy to American tennis in Melbourne following Monday's dreadful start.