MELBOURNE — Venus Williams and Sloane Stephens were upset early and the bleak opening day for Americans continued at the Australian Open on Monday with the women going 0 for 8 and two of the highest-ranked US men joining them as first-round casualties.
In her first match at Rod Laver Arena since losing last year’s final to her sister, Serena, Venus Williams lost her opener to Belinda Bencic 6-3, 7-5 and ensured the title won’t stay in the family.
Serena Williams hasn’t played a Grand Slam tournament since then because of her pregnancy and the birth of her first child.
For fifth-seeded Venus Williams, it was a first loss in five career meetings with Bencic, who lost to Serena in the first round last year.
‘‘I don’t think I played a bad match. She just played above and beyond,’’ said Venus Williams, who was playing her 18th Australian Open. ‘‘I just have to give her credit for that.
‘‘I didn’t play so bad, I wasn’t missing every shot. Just didn’t work out.’’
Bencic, who was born a couple of months after the last time there was no Williams in the second round at the Australian Open (1997), hit 32 winners, had 12 unforced errors and converted five of 11 break-point chances. Williams had 26 unforced errors and 22 winners.
Bencic credited playing with Roger Federer at the Hopman Cup — where they teamed up to win for Switzerland — with helping her recent improvement. She also had a new attitude against Williams, and she had Federer’s parents in the crowd for support.
‘‘I was thinking a lot about the last matches. For the first matches, I think I had a little bit too much respect, played a little bit careful and safe,’’ Bencic said. ‘‘This time I really tried to come out and hit it big.’’
US Open champion Stephens was the first American to fall, losing 2-6, 7-6 (2), 6-2 to No. 34-ranked Zhang Shuai. She was followed by Williams and No. 10-seeded CoCo Vandeweghe, a semifinalist here and at the US Open last year, who was feeling sick and frustrated in 7-6 (4), 6-2 loss to Timea Babos.
CiCi Bellis, the WTA’s Newcomer of the Year, was also among the eight US women who were out by mid-afternoon — 16 are in the singles draw.
No. 8-seeded Jack Sock, the highest ranked of the American men, lost 6-1, 7-6 (4), 5-7, 6-3 to Yuichi Sugita and his compatriot John Isner, seeded 16th, lost in four sets to Australian Matt Ebden.
‘‘Tennis is definitely a roller coaster,’’ said Stephens, who hasn’t won a match since her Grand Slam breakthrough last September. ‘‘But I have learned to just not panic. It will be OK.’’
No. 13-seeded Stephens had a chance to serve for the match in the 10th game of the second set but dropped her serve. She was outplayed in the tiebreaker and in the third set.
Ryan Harrison beat Dudi Sela in five sets and qualifier Mackenzie McDonald bucked the trend for the Americans.
Third-seeded Grigor Dimitrov — who beat qualifier Dennis Novak 6-3, 6-2, 6-1 — No. 6 Marin Cilic, No. 10 Pablo Carreno Busta, No. 23 Gilles Muller, No. 24 Diego Schwartzman, No. 28 Damir Dzumhur, No. 30 Andrei Rublev and No. 31 Pablo Cuevas all advanced.
US Open finalist and No. 11-seeded Kevin Anderson lost in five sets to Kyle Edmund.
The 20-year-old Bencic saved five break points in the eighth game before a rain delay caused an almost half-hour suspension of play as the roof was closed on Rod Laver Arena. She returned on a roll, winning the next six points to hold serve and then clinch the set.
Vandeweghe was leading in her first set before the rain delay on Hisense Arena, but sickness and irritation caught up with her.
She yelled an obscenity near the end of the second set and that, combined with a time violation in a changeover while she waited for a banana, led to a point penalty.
‘‘I felt like (Babos) was being quite in-your-face about her ‘Come ons.’ So it was more just explaining it to the umpire,’’ Vandeweghe said. As for the banana, she added: ‘‘I was just trying to wait for what I asked for to come and the chair umpire deemed that it wasn’t a good enough reason to wait for anything.’’
Of the other US women out in the first round, Sofia Kenin lost to No. 12 Julia Goerges — who extended her winning streak to 15 matches — Bellis lost to No. 30 Kiki Bertens, Alison Riske lost to Kirsten Flipkens, and Taylor Townsend lost to No. 19 Magdalena Rybarikova.
French Open champion Jelena Ostapenko made a positive start with a 6-1, 6-4 win over 37-year-old Francesca Schiavone, the 2010 French Open winner, and No. 15 Anastasia Pavlyuchenkova beat Kateryna Kozlova.
Olympic gold medalist Monica Puig saved a match point before recovering for a 4-6, 7-6 (6), 6-4 win over former US Open champion Sam Stosur.
Other seeded players falling included No. 24 Dominika Cibulkova, the 2014 Australian Open finalist who lost to Kaia Kanepi, No. 25 Peng Shuai and No. 31 Ekaterina Makarova.