Alize Cornet upset Andreescu for the second time in two weeks by beating the fifth-seeded Canadian 6-2, 6-1 in the first round; No. 9 Bencic fell 6-3, 6-3 to Kaja Juvan; and No. 4 Kenin's disappointing year hit another low when she was eliminated in the second round by Madison Brengle 6-2, 6-4.
Williams, meanwhile, was shut out in the second set during a 7-5, 6-0 loss to No. 21 Ons Jabeur.
With the defeats by Andreescu and Bencic, No. 6 Serena Williams being forced to withdraw midmatch Tuesday with a leg injury and No. 10 Petra Kvitova falling to Sloane Stephens on Monday, this matches the most losses by top-10 women's seeds in the first round during the Open era (since 1968) with the 2018 tournament.
This is also the first time since 1997 that no Williams sister will be in the third round at Wimbledon, the year before Serena made her debut at the Grand Slam.
Venus Williams was "bageled" in a set for the second time at Wimbledon, with the other coming in the second set of the 2017 final against Garbine Muguruza.
"Definitely not my best day. Maybe a few too many errors," Williams said. "But I think a lot of it was how she played. I think the courts really work for her. It was just great play on her behalf."
Williams had a tough time from the start, facing five break points in the 22-point opening game before she held. Playing for the second day in a row, the 41-year-old Williams seemed to tire, with only 15 winners and 36 unforced errors.
The on-court footing that knocked out Serena Williams continued to be an issue Wednesday, as Andreescu, Novak Djokovic, John Isner and at least one ball kid were among those taking tumbles, but all avoided serious injury.
"I didn't slip just once. I slipped like six times," Andreescu said. "The courts are super slippery. I have only played here once before, but they weren't like this at all. I spoke to a couple other players, and they said it's not that normal. But this is something we can't really control."
Andreescu, who has battled injuries, committed 34 unforced errors to seven for Cornet. Cornet, who also beat Andreescu in Berlin two weeks ago, broke the 2019 US Open champion five times and sealed the victory by chasing down a shot and hitting a lob that landed just inside the baseline on match point.
"I tried to stay positive the whole match," Andreescu said. "I tried to figure it out, how I can play her better. But honestly, she played really well."
Kenin struggled with her serve and had 41 unforced errors to seven by the steady Brengle, an American ranked 82nd who beat a top-10 opponent for the first time in more than four years.
Kenin, the 2020 Australian Open champion and runner-up at the French Open in October, is only 11-10 this year and lost in the second round of a major for the fifth time. The American also said Wednesday that she will not go to the Tokyo Olympics.
The 16th-seeded Pavlyuchenkova took advantage of 16 unforced errors from Bogdan to win in under an hour. She reached her first Grand Slam final at Roland Garros before losing in three sets to Barbora Krejcikova.
Elina Svitolina reached the second round with a 6-3, 2-6, 6-3 win over Alison Van Uytvanck. A Wimbledon semifinalist two years ago, Svitolina would play a potential third-round match Saturday with another victory -- the same day her country faces England in soccer's European Championship.
"I think there will be like two people watching my match," the third-seeded Svitolina said. "I'm completely fine with that. I'm just happy to be playing."
Sabalenka has a tour-best 31 match victories this year and is seeded second, but she is the only woman among the top 20 seeds who has yet to reach a major quarterfinal.
Boutler, ranked 219th, had the Centre Court crowd roaring until the end. But Sabalenka hit 10 aces and erased six break points to hold in one key game down the stretch.
Camila Giorgi, CoCo Vandeweghe and 19th-seeded Karolina Muchova also advanced to the second round. They were in among 27 first-round matches that had not been started or completed during the first two days because of rain delays that also created the slick conditions.
The Associated Press contributed to this report.