Halep broke Wang three times to race into a 4-1 lead before closing out the first set when she cut off the angle on a short forehand and blasted a winner into the open court.
But Wang came out firing in the second set, breaking Halep three times and holding to love in back-to-back service games to build a 5-1 lead.
The second seed stormed back but Wang nearly forced a third set when she quickly jumped out to 0-40 in the 10th game, only for the Romanian's confident net play to get her out of the jam.
Halep then went on to wrap up the win over 18th-seeded Wang in 89 minutes and match her best effort in Miami, which came in 2015 when she lost to Serena Williams in the semis.
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"Playing again in the semi-finals is going to be a big challenge. And maybe I can do my best result here and play in the final," Halep said on court. "It's going to be a big match in the next round."
Halep is the highest seed left in the draw following Osaka's third-round exit and she said that while she is aware she can reclaim top spot in the rankings it is not something she is overly concerned about.
"If I play well, if I play my best tennis, I have a better chance to become number one again," said Halep. "But it's not in my mind. I just want to find my rhythm, to find my level and just play as good as possible."
Halep will have her hands full when she battles the towering Pliskova on Thursday.
The Czech won six of the final seven games against the aggressive Vondrousova thanks to solid defence, an overpowering forehand and booming serve.
The fifth seed, who lost her last meeting with Halep, a three-setter at last month's Fed Cup, could potentially reach world number two by winning the tournament.
Recent form does not bode well for her, however.
"I practiced with Simona before the Miami Open and I lost 0-6," she said.
"So it can only get better."
(Reporting by Frank Pingue in Toronto and Rory Carroll in Los Angeles, editing by Ed Osmond/Peter Rutherford)
