Not the so-so serving. Or the bad backhands. This was a larger problem.
"Right now, she doesn't have her usual ability to respond and turn matches around," said Patrick Mouratoglou, who has worked with Williams since 2012. "It was obvious when she trailed 3-0 in the second set. Nothing happened."
Unable to get back on track once she no longer had control of the match, five-time Wimbledon champion Williams lost to 25th-seeded Alize Cornet of France 1-6, 6-3, 6-4 on Saturday in the third round, the latest in a recent series of surprising Grand Slam defeats.
"If I'm not playing a great, great match, these girls, when they play me, they play as if they're on the ATP Tour," Williams said, rolling her eyes.
She hadn't left Wimbledon so soon since 2005, also beaten in the third round. The No. 1-ranked and top-seeded Williams owns 17 Grand Slam titles, one fewer than Martina Navratilova and Chris Evert, but has departed before the quarterfinals at four of the past five majors. There were fourth-round losses at Wimbledon last year and the Australian Open in January, and a second-round loss at the French Open in May.
"It might be a bit premature to talk about her decline, but when she plays someone who finds the right tactics, she looks a bit lost on the court," Cornet said. "In my opinion, there are more and more players understanding how to play her."
Cornet also beat the 32-year-old American at the Dubai Championships in February, and watched video clips of that triumph before playing Saturday.
"I just knew that I could do it, because I did it once," Cornet said.
Still, this result was rather unexpected, given that Cornet never had been past the third round at Wimbledon, and only once before reached a major's fourth round.
"I cannot say that I played my best tennis today, really," Cornet said.
Perhaps, but it was good enough. On match point, after one last drop shot drew a netted response from Williams, Cornet pounded a fist on her chest, hopped around Court 1, then knelt to kiss the turf.
"It's very symbolic, because it means, 'Now I love you grass, and I didn't before,'" said Cornet, who had been 0-13 against top-20 opponents at majors.
Saturday's match was halted in the third game because of showers. When they returned about 4 1/2 hours later — "the rain delay killed me a little bit," Cornet said — Williams was terrific, reeling off five games to grab the first set.
Then everything changed, because Williams couldn't find the mark. She finished with 29 unforced errors, 11 more than Cornet. Two particular strokes let Williams down: Her serve, with seven double-faults and five breaks; and her backhand, with 12 unforced errors.
"I don't really know what I did wrong," said a blank-faced Williams, whose sister Venus lost Friday. "Usually I do. Usually I know I did this, this, and that."
It was by far the most significant outcome on a day full of starts and stops.
For his third match in a row, Nadal dropped the first set, this time beating 63rd-ranked Mikhail Kukushkin of Kazakhstan 6-7 (4), 6-1, 6-1, 6-1.
"All his shots just started to be much more hard, powerful and more aggressive," Kukushkin said. "It was like a different player."
Sharapova trailed 3-1, then won the next 11 games to top 44th-ranked Alison Riske of the United States 6-3, 6-0.
Federer, who has collected seven of his 17 major trophies at Wimbledon, never faced trouble and eliminated 35th-ranked Santiago Giraldo of Colombia 6-3, 6-1, 6-3.
Nadal next faces 19-year-old Australian Nick Kyrgios; Federer meets No. 23 Tommy Robredo; and No. 8 Milos Raonic, only the second Canadian man in the Open era to get to Wimbledon's fourth round, plays either No. 10 Kei Nishikori or Simone Bolelli, whose match was suspended because of darkness.
Cornet's fourth-round opponent when the tournament resumes Monday after the traditional middle Sunday off is No. 13 Eugenie Bouchard, a 20-year-old Canadian who was a semifinalist at the year's first two majors.
"It shows all of you who asked me if I'm ready to play Serena in Round 4," Bouchard said. "That's one of the reasons I don't look far ahead."