This is the third Grand Slam semifinal since 2000 to feature two first-time Grand Slam semifinalists.
WTA Staff

Simona Halep and Andrea Petkovic will face off at Roland Garros on Thursday in a rare instance of two first-time Grand Slam semifinalists contesting a Grand Slam semifinal. In fact, this will be just the third time this century that has occurred. Here are the circumstances of each:

2001 French Open SF
Kim Clijsters d. Justine Henin: 2-6, 7-5, 6-3
They would both go on to the very late stages of majors many more times, and quite a few times would face each other in Grand Slam semifinals and finals, but neither Belgian teenager had been beyond the fourth round of a major until the 2001 French Open. After prevailing in this clash against Henin, Clijsters would fall to Jennifer Capriati in a marathon final.

2009 US Open
Caroline Wozniacki d. Yanina Wickmayer: 6-3, 6-3
Wozniacki had not been beyond the fourth round of a Grand Slam until the 2009 US Open, where she had what remains her best result at a major. Fellow 19-year-old and World No.50 Wickmayer had not been beyond the second round of a Grand Slam until then and has not been beyond the fourth round since, but had a memorable run that fortnight. After getting through Wickmayer, Wozniacki lost to Clijsters in straight sets in the final.

2014 French Open
Simona Halep vs. Andrea Petkovic
Though she had plenty of success in other events, Halep did not start doing a lot of damage at majors until 2014. She had her first quarterfinal showing at the 2014 Australian Open and then followed that up by reaching her first Grand Slam semifinal at the French Open. She did not drop a set en route to the semifinals and lost just 22 games, beating the likes of Sloane Stephens and Svetlana Kuznetsova on the way. Petkovic had been to three Grand Slam quarterfinals in 2011 but had lost each of them. This time, she defeated Sara Errani in the quarterfinals to finally secure a Grand Slam semifinal berth.