In a rematch of last year's Wimbledon final, 22-year-old Muguruza once again came into the match as the underdog: Williams has won three out of their four previous meetings - all at Grand Slam level.
This time, the on-form Muguruza was not cowed by the occasion or by her World No.1 opponent, who was going for a record-equaling twenty-second major title.
"I think we both were very nervous," Muguruza told NBC's Mary Carillo after the match. "I was really going for the match so I was not really thinking of who I have in front or where I'm playing.
"I was just like, 'Come on, go for the match.' I just said [to myself], 'Garbiñe be calm, don't get nervous.' I practiced all my life for this so you know, that's the moment."
Muguruza stayed poised throughout the match's dramatic twists and turns, tamping down the nerves that have so often gotten the best of her in big moments. She earned the first break of the match for a 3-2 lead, then put a pair of double faults behind her to escape a 0-30 deficit and consolidate it. Williams broke back to level the score, but Muguruza stayed steady to earn a second break and serve out the first set 7-5.
The pair traded breaks to start off the second set, but Muguruza once again stayed composed and got her nose in front and built up a 3-1 lead. Williams fended off four of Muguruza's championship points on her serve at 5-3, putting up a monumental effort to hold her ground and win a 16-point game to force the Spaniard to serve for the match. And the No.4 seeded Muguruza did: she served it out at love and claimed the match on a backhand lob winner to close out Williams, 7-5, 6-4.
With the win Muguruza not only claims her first Grand Slam title, she also adds her name to Spain's storied history at Roland Garros: she's the first Spanish woman to win the title since Arantxa Sanchez-Vicario accomplished the feat in 1998.
Muguruza also climbs two spots in the rankings to World No.2, matching another one of Sanchez-Vicario's feats by becoming the first Spaniard to hold that ranking since 1996. She sits behind Williams, who retains her No.1 ranking.
Both players were understandably emotional during the awarding of the Coupe Suzanne Lenglen, presented by WTA founder Billie Jean King and French Tennis Federation president Jean Gachassin. Williams fought back tears as she delighted the Chatrier crowd by delivering her runner-up speech in perfect French, while Muguruza had only praise for Williams, a player she grew up admiring.
"I can't explain with words how this day means to me. You work all your life to get here," she said.
"I want to really congratulate Serena because she's one of the best players."
For Muguruza, a 22-year-old Venezuelan-born Spaniard of Basque heritage, the victory is not just for Spain but for every part of her multicultural background:
"I've grown up playing on clay so for Spain and for me this is just amazing," she said. "I know [tennis] is very traditional in Spain, but Venezuela is in my heart also, I also play for them."
? WTA (@WTA) June 4, 2016