Also through were Svitolina and e Mertens, providing some comfort for tournament organizers who have somehow ended up with three of the world's top 10 in the last four, despite a high number of prominent withdrawals.
The players traded breaks in the third set, but Mertens broke in the fifth game of the final set to make it 3-2 - the 12th break of serve in 24 games in the match.
"I couldn't say I was really nervous", she added, "but I think my shots went long". "I am very happy, honestly", said Halep.
Seemingly down and nearly out at 1-4 in the third set, world number three Halep was roared on by a large and noisy set of Romanian fans to secure a remarkable 6-3, 3-6, 6-4 victory over the Ukrainian.
Simona Halep won five games in a row in the final set to beat Elina Svitolina in the semi-finals of the Qatar Open. Their last meeting took place in the fourth round of the 2018 French Open, where Halep sailed through to the next round in straight sets. That being said, this is expected to be a tough challenge for Mertens as she is playing one of the elite players in the tennis world.
With tears streaming down her cheeks, Mertens shook hands with Halep and then quickly darted towards her coach David Taylor to celebrate the biggest win of her young career.
"I hope I can come back here for many more years", she added with a smile.
Hitting with pace and depth, the Ukrainian grabbed another break in game nine to close the set and set a decided where she had the advantage on her side. But I m happy that I won it.
She will play Belgium s Mertens, the world number 21, in the last four, who beat Kiki Bertens of the Netherlands 6-4, 6-3.
On the other side of the coin, Angelique Kerber enters this match having won her previous match in 3 sets. Mertens saved six out of nine break points to limit the damage in her games, dominating in the deciding set to join Simona in the title match. After winning three singles titles last season, she is now on the path to achieving something better - win her first Premier tournament.