Muguruza's victory in Cincinnati catapults her to No.3 in the WTA rankings, just 530 points behind Pliskova for No.1.
While neither was playing for the No. 1 spot like Halep in the first match, there was plenty at stake in the ATP final. Nadal could meet Australian Nick Kyrgios in Friday's quarter-finals.
Ukraine's fourth-ranked Elina Svitolina lost her chance at number one when she fell 7-5, 6-4 to Germany's Julia Goerges, who then lost to Stephens 6-1, 7-6 (7/3).
"I played so bad today and Garbine played so well", Halep, told the crowd.
The Spaniard came through 6-3 6-2, having set the tone after breaking the Czech in the very first game.
'Honestly, I was thinking in her situation, it must be hard, ' Muguruza said. "And I feel, like, s--". As her head began to drop, especially after failing to break at 0-3 down in a long game with her only break point opportunities of the match, it really became a foregone conclusion. The Aussie served for the match in the 10th game but double faulted on his third match point and Nadal broke back.
"I'm obviously excited but it's just another tennis match for me", Kyrgios said. I battled back and I'm glad things went my way."Muguruza has a last-eight date with eighth seed Svetlana Kuznetsova". "After a few games I started to go down in confidence. I had some chances in the second set", Pliskova said. Muguruza, on the other hand, won 68 percent of her first serve points and 69 percent of her seconds. The 20-year-old has spent the year since Rio de Janeiro taking a break from the sport and says she is in no hurry to make any sort of concrete decision. I played three tournaments in a row. "It was just not a great day". Muguruza knocked off the defending champion, Karolina Pliskova, 6-3, 6-2, in the semifinals.
Kyrgios returned to complete a last-16 win over Ivo Karlovic, while Nadal - set to return to the ATP summit on Monday - outclassed Albert Ramos Vinolas in their rearranged fourth-round tie. But I want to be in her position. "I think that that was the key". I knew I was going to be in danger at some point, so I stayed calm. "So finally, this year, you know, I improved that". "These moments are bringing you a lesson and I'll just learn from it". "I think we both played very well, especially in the third set".