The victory also completed a historic day for Japanese tennis with Naomi Osaka routing Ukraine's Lesia Tsurenko 6-1 6-1 and the pair became the first Japanese man and woman to reach the semi-finals of the same grand slam.
Keys, who fired six aces and fended off both break point opportunities against her during the match, told reporters that adapting to Suarez Navarro's versatile play proved critical.
After a strategic shift in which Williams turned into a serve-and-volleyer, she ran off 12-of-13 games to close out Latvia's 19th seed Anastasija Sevastova 6-3, 6-0 in Thursday's U.S. Open semifinals. She then stunned defending champion Sloane Stephens 6-2 6-3 in the last eight.
Williams faces the victor of Madison Keys-Naomi Osaka in Saturday's final.
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She had also captured the prestigious Indian Wells title in March and defeated Serena Williams in Miami.
"Well, in the mornings I don't play matches, I walk around and hear the lovely honking", Osaka said.
The match was a repeat of the 2014 US Open final, which Cilic won, and the Croatian made a strong start as he took the opening set. She recovered to win in three sets, with her opponent accusing her after the match of acting.
Cilic has reached two Grand Slam finals in the past 18 months and was eager to make up for the disappointment of a second-round exit at Wimbledon.
The Latvian's own serve was landing legally at a respectable 60% coming into the semi-final, but it dipped here and her reliance on placement over speed was exposed when Williams broke back in the fourth game, and again for 4-2. She's lost only one set in the tournament to this point.
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It was another hot afternoon Wednesday, with temperatures in the high 80s but feeling about 10 degrees hotter with the humidity.
After 40 minutes, Williams bullied her way artfully to take the first set, reading Sevastova's grip switch for the slice with uncanny accuracy, which allowed her to grab time for a dash to the net. "I didn't want to show this kind of game in front of this big crowd, but unfortunately I'm just not able to play now".
He is into his third major semifinal, all in NY but is still in search of his initially Slam trophy.
"She does a really good job. of holding the baseline and redirecting the ball", Keys said. "I still haven't totally processed this match". He made the best possible start on another scorching hot day in NY by racing through the opening set in 36 minutes.
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