Top seeds Caroline Wozniacki and Simona Halep reached the quarter-finals of the Qatar Open, but Thursday was not without its shocks as defending champion Karolina Pliskova was dumped out.

Australian Open champion Wozniacki - a finalist here last year - and Halep got the better of Monica Niculescu and Anastasija Sevastova, respectively, while18-year-old CiCi Bellis earned an unlikely win over Pliskova in straight sets.

The tournament's other seeded players were not so easily defeated, however, with Garbine Muguruza, Caroline Garcia and Julia Goerges avoiding upsets to join Wozniacki and Halep in the last eight.

WORLD TOP TWO HEAD THROUGH

Wozniacki and Halep are on opposite sides of the draw and they remain on course for a rematch of last month's Australian Open final after a pair of last-16 wins.

It looked at once stage as though Wozniacki could be in trouble, however, blowing a 4-1 lead in the first set to trail 5-4.

The world number one was required to break Niculescu as she served for the set, but she then made sure there would be no jitters later in the match and dispatched of the Romanian.

Halep's triumph was much more straightforward, although she too faced a tricky opponent in Sevastova and had to show her class to emerge 6-4 6-3.

TITLE DEFENCE HALTED BY BELLIS

Halep will now face an unseeded opponent in the quarter-finals after Bellis stunned defending champion Pliskova on centre court.

The American teenager had come from behind to see off compatriot Madison Keys in the previous round and again showed her promise to secure a first top-five scalp by seeing off Pliskova 7-6 (7-4) 6-3.

Bellis had no interest in celebrating her achievements, though, quickly turning her attention to Halep.

"I think this is definitely a milestone; it's great," she said. "But now I move on to the next one and just start thinking about [Friday]."

SEEDS POPULATE THE LAST EIGHT

The unseeded trio of Sorana Cirstea, Anna Blinkova and Mihaela Buzarnescu could not match Bellis' feat, with respective victories for world number four Muguruza, Garcia and Goerges.

Petra Kvitova landed the other standout result of the round and will take on Goerges, having impressed as she knocked out world number three Elina Svitolina 6-4 7-5.

The Czech 16th seed has now won her last 10 matches.

Meanwhile, Wozniacki will face Angelique Kerber, who edged out Johanna Konta 1-6 6-1 6-3 in a thriller late on Thursday.

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