However, once again Stephens proved to be simply far too solid for her good friend, saving break points early on before breaking once in the first set, and then surviving a slight wobble towards the end of the second set to seal a 6-4, 6-4 win in an hour and 17 minutes.
Halep has not lost to Stephens in five years, winning the pair's past four encounters in straight sets.
She won the US Open in September on a ranking of 83 and just 69 days after returning to the tour following her near year-long absence.
The friends hugged at the net, both smiling.
Coached by Darren Cahill, Halep will bid to provide the South Australian with his third grand slam champion.
"To have two Americans in the semifinals of the French Open, I think, is pretty incredible", said the 25-year-old.
Juan Martin del Potro is still trying to figure out which spectator distracted him when he was about to serve.
Cilic made no mistake when the second match point finally came in the fifth set.
He returned to the court with his leg bandaged and lost the next game. "And when I got my opportunity to break, I was like, 'It's go time'".
The imposing Argentine then walked over to the side of the court and began eye-balling a row of spectators.
"They obviously both move really well". So like I said, making the quarter-finals here was a big deal for me.
"I think that's a load of crap, but, you know, it's just me".
He has no trouble identifying his semifinal opponent: Rafael Nadal.
Top-seeded Simona Halep advanced to the French Open quarterfinals for the third time by beating Elise Mertens 6-2, 6-1.
Halep feels better equipped to finally break her duck in Paris.
"Honestly, the Open feels like it was 12 years ago at this point", the 13th seed said.
Madison Keys vowed to carry on smiling after moving closer to a maiden grand slam title. Along the way, Simon defended the status of the first racket of the world and virtually guaranteed a place at the Final tournament. This is Robinson's 19th French Open for NBC.
"I don't know if I am going to play on grass", said the Serb.
Serena Williams has called off her Grand Slam comeback, pulling out of the French Open because of a chest injury before she was supposed to play Maria Sharapova.
Either she or Sloane Stephens, the 2017 US Open victor, is now set to depart this year's French Open as the top-ranked American on the court - a title that only three different players have owned since 2007.