Will it be the seemingly indifferent Williams who steps onto Court Philippe Chatrier for the French Open final -- or the hard-hitting, steely Serena?
Williams is looking for her fourth Roland Garros singles title and her 22nd overall in grand slams, which would equal the record of Steffi Graf and place her two behind all-time record holder Margaret Court.
After her semi-final win against Dutchwoman Kiki Bertens, Williams did not seem overexcited at the prospect.
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"I guess it's how you look at it. The same with 21 and trying to get to another one. Nothing I can do about it," she told reporters. "The only thing can I do is just play to win the tournament and that's it."
While the American looks in a different league on paper, that may not be the case on court.
If the blips Williams suffered en route to laboured wins in her last two matches resurface, the 22-year-old Muguruza is well placed to take advantage.
The fourth seeded Spaniard was the last player to beat Williams at Roland Garros, crushing the American 6-2 6-2 in the second round in 2014 for her only win in four encounters.
"It was a really unbelievable lesson for me. It propelled me to many, many, many wins after that," said Williams.
The 34-year-old, who hopes to become the first woman to retain her title here since Justine Henin in 2007, won their grand slam confrontation 6-4 6-4 at Wimbledon last year in Muguruza's first appearance at that level.
The Venezuelan-born player says she has been on a learning curve since then.
"I have learned a lot how to control my emotions inside the court and outside the court," said Muguruza, who hopes to become the first Spaniard to lift the Suzanne Lenglen Cup since Arantxa Sanchez Vicario in 1998.
Failure to take a deep breath at key moments may have been a factor at Wimbledon last year.
"It was difficult for me to manage stress. I remember three points that I missed and she took advantage of this, but otherwise I wouldn't say I made a very big mistake," she said.
She may also draw hope from the fact that Williams could be suffering from a groin problem. Asked if she was injured, Williams said: "I have had some issues, but, you know, it is what it is."
(Reporting by Julien Pretot; editing by John Stonestreet)
