Former world number one Angelique Kerber recovered from losing the first set to beat Britain's Johanna Konta and reach the Qatar Open quarter-finals.
Tenth seed Konta, 26, raced into a 5-0 lead against the eighth seed and won the opening set in 37 minutes.
But the 30-year-old German took the next in 28 minutes and secured an edgy decider in Doha to win 1-6 6-1 6-3.
She will play world number one Caroline Wozniacki in the last eight after the top seed beat Monica Niculescu 7-5 6-1.
Having lost her most recent meeting with Konta on the grass at Eastbourne last summer, Kerber could not contain the Briton in the early exchanges and the world number 11 converted her sixth set point.
However, world number nine Kerber soon took command, wrapping up the match in one hour 51 minutes and maintaining an unbeaten record in three matches on hard court against Konta.
"I think in the first set I needed a bit of time to find my rhythm," Kerber said. "It's always tough to play against Johanna, she's such a good player so I was trying to go for it in the third set."
Wozniacki, meanwhile, committed 18 unforced errors against world number 92 Niculescu and complained to French umpire Emmanuel Joseph during the match about the Romanian's grunting.
"It isn't in the rule book that you are not allowed to grunt when the opposing player hits?" the 27-year-old Dane asked.
In a courtside interview after her victory in one hour 30 minutes, Wozniacki said of 30-year-old Niculescu, who beat Maria Sharapova in the first round, "she tries to get in your head".
Defending champion beaten by teenage qualifier
Elsewhere, unseeded 18-year-old American qualifier Catherine Bellis beat defending champion and fifth seed Karolina Pliskova 7-6 (7-4) 6-3 in an hour and 36 minutes.
The world number 48 broke Pliskova's powerful serve five times and it was the teenager's first success against a player in the world's top five.
She attributed part of her success down to eating ice cream every night in Doha, which "gives me a bit of energy".
Bellis will play second seed Halep in the quarter-finals after the Romanian saw off Anastasija Sevastova of Latvia 6-4 6-3.
Third seed Elina Svitolina of Ukraine was beaten 6-4 7-5 by former Wimbledon champion Petra Kvitova, seeded 16.
Left-hander Kvitova, 27, has won her past seven meetings with Svitolina and since a first-round defeat to Andrea Petkovic at the Australian Open, the Czech has now won 10 consecutive matches.