Former champion Maria Sharapova went out in the first round at Wimbledon after she was forced to quit her match against France's Pauline Parmentier due to an injury.
The Russian trailed 4-6 7-6(4) 5-0 before she conceded the contest.
Garbine Muguruza, Wimbledon champion in 2017, was knocked out in the first round, losing 6-4 6-4 to Beatriz Haddad Maia.
The world number 27 was dumped out by the 23-year-old Brazilian, who has never won a career singles title and is ranked 121 in the world.
The defeat added to a barren spell at the majors for the 25-year-old Muguruza, who has not been past the fourth round in any of the last five Grand Slams.
The defeat also continued a patchy season for the Spaniard who has been past the quarter-final at only one tournament, when she won in Monterrey in April.
The perfect start to her title defence…
— Wimbledon (@Wimbledon) July 2, 2019
Last year's champion @AngeliqueKerber beats Tatjana Maria in straight sets to secure her spot in the second round#Wimbledon pic.twitter.com/ekmbCH1eCf
Defending champion Angelique Kerber needed to call upon all her cunning and experience to navigate her way 6-4 6-3 through an awkward first-round encounter with German compatriot Tatjana Maria.
The three-times Grand Slam champion has enjoyed mixed fortunes since beating Serena Williams in the final last year, dropping to fifth in the standings, and went out in the first round of the French Open last month.
It was with some nervous tension that she stepped out on Centre Court, opening the action on the second day of the tournament in traditional fashion as the women's champion.
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Maria's combination of slice and guile, rare on a circuit dominated by big-hitting power players, frustrated Kerber, especially in the first set.
"She's playing tricky on grass," Kerber said straight after leaving the court. "I was hoping to stay in the match because you never know what's going to happen with her. She really plays tricky with the slice on both sides."
Cruising through…
— Wimbledon (@Wimbledon) July 2, 2019
World No.1 @ashbar96 eases into the second round at #Wimbledon with a comfortable victory over Saisai Zheng pic.twitter.com/Q4zcZqsgqN
Ash Barty's express-train run to the top of the women's game shows no sign of slowing as the world number one dispatched China's Zheng Saisai 6-4 6-2 to reach the second round.
Australian Barty, who followed up her French Open triumph last month by winning in Birmingham, sealed a 13th straight win in a brisk hour and 16 minutes, battling through the first set before easing through the second.
Barty has never been past the third round at Wimbledon, but her game is well-suited to the slick grass courts and, having dragged herself to the summit of women's tennis, she looks well-placed to challenge for a second Grand Slam title.
While some might feel the weight of expectation coming into a Grand Slam as world number one, it seems Barty is relishing the experience.
"It feels incredible. It feels a bit bizarre actually but I'm trying to go about my business in the same way as I always have done," she said.
"You have to enjoy every minute when you're playing at this beautiful tournament."
British number one Johanna Konta battled into the second round with a 7-5 6-2 victory over Romanian qualifier Ana Bogdan.
She said: "It was very tough out there. I don't think either of us felt quite comfortable, we were both trying to find our feet.
"First rounds are never easy and I'm really pleased to have competed through. So far in the season I feel like I'm building in every match I'm playing."
Ex-US Open champion Sloane Stephens won her first match at the All England Club since 2016, beating Timea Bacsinszky 6-2 6-4 on Court 2.
Serena Williams' next job will be to decide whether to partner Andy Murray in the Wimbledon mixed doubles after coasting through her opening singles match.
The American and the Scot talked up the idea of joining forces during their pre-tournament press conferences at the All England Club on Saturday.
And having started her quest for an eighth singles title at SW19 with a 6-2 7-5 victory over Giulia Gatto-Monticone, it is now crunch time for the 37-year-old with the deadline looming on Wednesday at 11am.
More to follow...
