-- She blew through the formidable field in Miami back in March, hacking down Angelique Kerber, Maria Sharapova and Li Na -- the last two her most formidable rivals this year -- in swift succession.
And then the Sony Open champion traveled up the road to Charleston and was stunned in her first match by Slovakia's Jana Cepelova, the WTA's No. 78-ranked player.
At 32, Serena Williams continues to be the best women's player in the game. Her serve, considered by many the best ever, is still lethal. Her quickness and top-end speed, when she is healthy, remains crackling. In recent years, her fitness and tennis acumen have actually improved.
But there are times out on that court when she's not fully engaged. Heading into the French Open, which begins Sunday, this is the prevailing question on the women's side: Will the defending champion step onto the red clay of Court Philippe Chatrier fully present in both body and mind?
Or will she suffer a brain cramp and lose to someone like Cepelova? Or Virginie Razzano, who stunned her in the first round at Roland Garros two years ago? Or Ana Ivanovic, who back in January beat her in the fourth round of the Australian Open?
Serena has sensational records against the truly elite handful of players who chase her No. 1 ranking position. In the end, it will likely come down to a single factor: Serena Williams herself.
"I think with Serena, it's up to her," said Darren Cahill, who coached No. 1 players Andre Agassi and Lleyton Hewitt. "That's the great thing about it: She gets to choose.
"When she wants to play, she's tough to beat. She treated Miami like a major and she won it. You could see from the first ball, she was serious. In Charleston ... well, not so much."
When she arrived in Madrid, Serena admitted she had been tired in Charleston.
"I'm so much better," she said. "I took so much time off. I needed it.
"I'm good now."
But she wasn't. Williams suffered an injury to her left thigh in a first-round match and, after playing two more matches with it heavily taped, withdrew before her quarterfinal with Petra Kvitova. That was 16 days before the balls start flying at Roland Garros.
"I just need some time to rest and recover," Serena said afterward. "It's beyond words. It's so frustrating."
Maybe age has caught up with her. Or maybe she's just sandbagging. Whichever it is, even with her title in Rome last week, the French appears to be a little more Open this year.
"Yes," said Steve Flink, a 40-year veteran of the game and a TennisChannel.com columnist, "it's more wide open than in a long time. Last year Serena was up for it, but I'm not nearly as sure of her this year. I could see a surprising loss in the round of 16.
"If that happens, I could see six or seven players with a chance. Sharapova winning in Stuttgart was an important step. She's had two great Frenches in a row. I give her the next-best chance after Serena. Li Na has proven she can be consistent on clay, too."
Making the transition
Sometimes, for power players, clay can feel like quicksand.
The soft dirt diffuses their heavy shots and running down balls can feel like a day at the beach -- but not in a good way. In theory, big players like Williams, Sharapova and Victoria Azarenka are at a disadvantage on clay.
In reality, once they solve the movement issue, they are nearly as dangerous on the dirt.
"That's why I love clay so much -- you have to move beautifully on it," said Mary Carillo, who will work as an analyst at the French Open for both NBC and Tennis Channel. "[Rafael] Nadal, sure, he has the power and spin, but his movement is the key. This has been borne out more today than at any other time in recent tennis history. It's why Nadal has dominated and [Novak] Djokovic is close.
"Serena is balanced better over her feet now. Maria figured out how to move on it, too. Maria, obviously, can play on clay. She's a dirt-baller now."
Williams won 28 consecutive matches on clay last spring and five titles, in Charleston, Madrid, Rome, Paris and Bastad. She beat Sharapova in two of those finals and Azarenka in another.
This year, the clay circuit has been No. 7-ranked Sharapova's private sandbox. With Williams out of the picture, Sharapova vaulted to the title in Stuttgart and a week ago defeated Simona Halep in the Madrid final 1-6, 6-2, 6-3. Heading into Rome, she had won 11 straight matches on clay and 46 of the past 49 -- with all three losses coming to Williams.
"I really challenge myself to improve on clay courts," Sharapova said in Madrid, "because that was never my favorite surface in the beginning of my career. I've done a really good job of transitioning from hard to the clay and really improving physically and recovering well from match to match. I think that's helped me a lot. I've benefited from that in the last couple of years."
Sharapova has lost 16 of 18 head-to-head meetings with Serena -- including all four on clay -- and an amazing 15 in a row. But when Serena checked out early at Roland Garros in 2012, Sharapova claimed her first French Open title and completed a career Grand Slam.
According to Williams, there are more players with a legitimate chance to win than there used to be.
"I work hard, but there are other people who work hard, too," she said in Madrid. "If not, there wouldn't be so many good players. I think consistently there is more power now. I think 10 years ago, there were probably six or seven players who had a lot of power. But I think now there's more."
But though Roger Federer, who is the same age as Serena, routinely has difficulty dispatching the men at the top of tennis -- Nadal, Djokovic, Andy Murray and now even Stan Wawrinka -- when there's a good player in front of her, Serena usually wins.
Consider the head-to-head record against Serena for the nine players below here in the WTA's top 10: Li (2-10), Agnieszka Radwanska (3-15), Azarenka (3-15), Halep (0-3), Kvitova (0-5), Jelena Jankovic (4-8), Kerber (1-4), Sharapova (2-16) and Dominika Cibulkova (0-4).
It adds up to a composite 12-73 mark.
"It's a confusing time in women's tennis," Carillo said, "but I'm actually encouraged by some of the women. There are some gritty players who stand toe-to-toe on the baseline and think they deserve to win. That's what's been missing from women's tennis for years. There's been a nice little sea change there."
A two-horse race?
So going in, we're looking at Williams and Sharapova -- not necessarily in that order -- as your favorites for the French title they each have won once in the past two years.
"Maria can't play with Serena," Carillo said, putting her finger on the critical issue, "but if she doesn't have to, you can't rule her out."
Nick Bollettieri, who will be enshrined in the International Tennis Hall of Fame after Wimbledon, coached them both. Sharapova arrived at his academy in Bradenton, Florida, from Siberia when she was 9 years old; Serena has also sought his advice over the years.
"I've had the privilege of working with both of them," Bollettieri said the other day from Florida. "After Maria won Stuttgart, I sent her manager Max [Eisenbud] a note. I said if she needed a chauffeur to drive all those Porsches, I'd sign up. Maria is playing great on clay right now.
"Like Maria, Serena can slide on that stuff now, but remember, no matter what surface, that big serve makes a difference. On the other end, give Serena any defensive serve, it's Katie, bar the door. She takes advantage of those opportunities, no matter [what] surface it is."
So who's in the best position to win?
"The field is wide open," Bollettieri said. "But I'll tell you this: I've never said this, but I believe Serena is the best player in the history of women's tennis. If she stays healthy and finds the desire to keep winning, that's just what she'll do.
"If she drops out, it's a tremendous hole that would open up in the draw for women's tennis."
Pam Shriver, a 21-time Grand Slam doubles champion and ESPN analyst, isn't discounting Sharapova's chances.
"To me, she has a chance for her second French Open," Shriver said from California. "Even Serena is a question mark. She hasn't proven she's in her best shape, but she's always in that mix. Maria and Serena -- if you'd said they were the French favorites five years ago, nobody would have believed it.
"Somebody's going to give Serena a run for her money. I don't believe she can win two in a row. I think she's going to get upset along the way."
For Cahill, Serena should be looking at the broader picture -- far beyond Charleston, Madrid or even Rome.
"Going forward," he said, "I think she's going to focus more on the Slams. She's got to get her schedule to the point that she's ready to play every time she goes out. That might be only a dozen events a year, but you want to see her excited about playing."