Struggling Halep knocked out of Wuhan Open

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World number one Simona Halep injured her back during the Wuhan Open

World number one Simona Halep was the biggest casualty on a day of upsets at the Wuhan Open in China on Tuesday, with the Romanian toppled in straight sets by Dominika Cibulkova.

The sparse crowd watching the late-night clash was stunned as the decidedly off-colour Halep failed to win a single game against the hard-hitting Slovakian in the first set.

The French Open champion suffered a back injury during a practice session on Sunday, and failed to fully recover for her opening match in Wuhan against the 31st-ranked Cibulkova, despite two days bed rest.

"It was tough at the beginning to play... (but) I didn't want to (retire) and I just pushed myself to play better and better," Halep said.

Despite the injury and needing treatment several times during the match, she put up a strong fight in the second set, breaking Cibulkova's serve three times before eventually succumbing 7-5.

Even with the injury, "I was close in the second set, so this match just gives me confidence that my level is really, really good", said the top seed, who was cheered every time she ran back after treatment to resume play.

Halep, playing her first match since her early exit from the US Open last month, added that she expects to recover from the back injury "in a few days".

She was the biggest casualty on a day of upsets at the 2018 Wuhan Open, a $2.7-million tournament which features nine of the WTA top 10.

World number four Caroline Garcia was knocked out by qualifier Katerina Siniakova in a gruelling clash, with the second and third sets decided by tie-breakers.

Earlier, local favourite Wang Qiang ousted the 7th-ranked Karolina Pliskova 6-1, 3-6, 6-3 in another thriller while 6th-ranked Elina Svitolina also failed to impress, going down 6-4, 2-6, 6-1 to Aryna Sabalenka.

- Kerber, Wozniacki advance -

It was a much happier day for Wimbledon champion Angelique Kerber and Australian Open holder Caroline Wozniacki, both proceeding comfortably to the next round.

Wozniacki, who received a first-round bye, was barely troubled by the 61st-ranked Swede Rebecca Peterson, winning 6-4, 6-1 to set up a third-round clash with Olympic champion Monica Puig, who romped past Aliaksandra Sasnovich 6-2, 6-2.

World number two Wozniacki is looking for a good run in Wuhan to secure a return to the WTA Finals, which she won last year.

"It's great to get a good start, obviously. Every match counts," Wozniacki said.

"I'm just trying to keep my head down and keep focused and try to win as much as I can to get that (WTA Finals) spot."

The 3rd-ranked Kerber -- along with Halep -- has already qualified for the Finals, which will be held in Singapore next month, and advanced after a knee injury forced her American opponent Madison Keys to retire.

The German looked set for a big win over the 2017 US Open semi-finalist and was leading 6-0, 4-1 when Keys could not continue.

Kerber could return to world number one with a successful end to the season, and the chances of that have been boosted by Halep's wobble in Wuhan.