After winning two straight Grand Slam titles, Naomi Osaka has now failed to reach the second week at the last two majors.
Osaka, who was ranked No. 1 in the world until last week, lost to Yulia Putinseva 7-6 (4), 6-2 on Monday in the first round at Wimbledon.
The second-seeded Osaka won the U.S. Open last year and the Australian Open this year, but she lost in the third round at the French Open and now the first round at the All England Club.
"I just don't think I played that well," said Osaka, who also lost to Putinseva two weeks ago in Birmingham. "But I wasn't surprised because I've played her, like, twice already."
Osaka had 38 unforced errors on Centre Court, while Putinseva had only seven.
Osaka later cut short her news conference, leaving before any questions were asked in Japanese.
"Can I leave?" she asked the moderator after several short answers. "I feel like I'm about to cry."
There were two big upsets on the men's side as well with both sixth-seeded Alexander Zverev and seventh-seeded Stefanos Tsitsipas losing in the first round.
Zverev lost to Jiri Vesely of the Czech Republic 4-6, 6-3, 6-2, 7-5, while Tsitsipas was beaten by Thomas Fabbiano of Italy 6-4, 3-6, 6-4, 6-7 (8), 6-3.
Both Zverev and Tsitsipas are among the younger generation of players expected to eventually replace the Big Three of Roger Federer, Rafael Nadal and Novak Djokovic in men's tennis.
Tsitsipas, who beat Federer at the Australian Open en route to the semifinals, saved two match points in the fourth-set tiebreaker and failed to convert three break points early in the fifth. Fabbiano broke Tsitsipas twice in the final set, including in the last game when the Greek popped a shot long.
It was the 102nd-ranked Fabbiano's first win over a top-10 player.
Zverev, who lost in the first round at the All England Club for the first time in five appearances, said his confidence was "below zero" right now.
"It was kind of a typical Grand Slam match for me. I started off well, then one or two things don't go my way, and everything kind of a little bit falls apart," the German said. "Yeah, I'm not very high on confidence right now."
Earlier, last year's Wimbledon men's finalists both advanced to the second round on the opening day of this year's tournament.
Defending champion Djokovic was first on Centre Court, as is tradition at the All England Club. The four-time champion beat Philipp Kohlschreiber 6-3, 7-5, 6-3 in the first round.
"It's a sacred court, the cradle of our sport for sure," Djokovic said in the tunnel after walking off the grass. "It has a very special place in my heart, in my career as well.
"I've been blessed to be very successful on this court over the years so every time I step on it memories come back and a great feeling."
Djokovic started the match in a hole, with Kohlschreiber breaking in the top-seeded Serb's opening service game. But the troubles ended there for Djokovic, who had lost to Kohlschreiber in Indian Wells on a hard court this year.
"I know that he's capable to play some really good quality tennis on various surfaces, especially on grass," Djokovic said. "He takes the ball early, has a good slice, good serve. So it was good test for me."
Fourth-seeded Kevin Anderson, who lost to Djokovic in straight sets in last year's final, moved into the second round by beating Pierre-Hugues Herbert of France 6-3, 6-4, 6-2 in only his third match since March.
"Different expectations coming in right now given that I've not played many matches this year," said Anderson, a South African who also reached the 2017 U.S. Open final. "But I think there's a lot of positives I can take from today, and hopefully I'll be able to keep building on that."
Stan Wawrinka, seeded 22nd, also advanced. The three-time Grand Slam singles champion, who has won each of the other three majors but never Wimbledon, defeated Belgian qualifier Ruben Bemelmans 6-3, 6-2, 6-2.
Wawrinka, who will next face tall American Reilly Opelka in the second round, has never been past the quarterfinals at the All England Club.
"I think for sure some tough years here by losing first or second round. I had two good years by making quarterfinals," Wawrinka said. "I do believe that this year I'm playing well, I playing the best I never played on grass court. For sure I am having a tough next match."
The 21-year-old Opelka, who is 6-foot-11 (2.11 meters), beat Cedrik-Marcel Stebe of Germany 6-3, 7-6 (4), 6-1.
Simona Halep, a former No. 1 on the women's tour who is seeded seventh at Wimbledon, advanced despite some pain in her left knee and foot. The Romanian, who called for a trainer after winning the first set, trailed 5-2 in the second set before rallying to beat Aliaksandra Sasnovich 6-4, 7-5.
Third-seeded Karolina Pliskova also advanced, beating Lin Zhu 6-2, 7-6 (4).
Later Monday, 39-year-old Venus Williams was playing 15-year-old Coco Gauff.
Williams is the oldest player in the women's field. Not only is Gauff the youngest, but the American is also the youngest player in the professional era to reach the main draw at the All England Club by going through qualifying rounds.
Williams had won four of her seven Grand Slam titles by the time Gauff was born.