Adelaide, Jan 15 (AFP) Angelique Kerber's preparations for the opening Grand Slam of the year were dealt a blow Wednesday when she was forced to retire from the Adelaide International with back pain.

The German, a former world number one who won the Australian Open in 2016, pulled out when behind 6-3, 2-0 in her second-round clash with Ukraine's Dayana Yastremska.

The ninth seed began showing serious signs of the back problem early in the second set then stopped after two points in the third game on her own serve, calling for the trainer.

A medical evaluation on court ended with the 31-year-old world number 18 calling it quits against her teenaged opponent.

Yastremska kept calm during the delay, reading what she called "secret" notes.

"I didn't want to lose focus," said the world number 24, who has won three titles over the past two seasons.

On other courts, Croat Donna Vekic rallied past Maria Sakkari of Greece 2-6, 7-5, 6-1, winning in two hours and overcoming six aces while breaking four times.

She moved into a quarter-final against Yastremska.

In the men's draw, Russian Andrey Rublev, who the Qatar Open at the weekend, began his Australian adventure by beating American Sam Querrey 6-3, 6-3 to move into a last eight clash against Britain's Dan Evans.

Rublev, who missed six weeks in 2019 with a wrist injury, improved to a perfect 5-0 for the season.

The Russian did not let a delayed flight from Doha disrupt his on-site training in Adelaide, with the third seed ready to go against Querrey.

"It's important that I have mental confidence," he said. "One day you still can play so good, the other day can be not that good.

"At the end it's all about how you accept these things and how you will find the solution if you're not playing well."

Rublev's win in Doha moved him up to number 18 in the world.

"It's an amazing feeling to be top 20 for the first time, but there are still so many things that I can improve," he said.

South African qualifier Lloyd Harris also advanced to the quarters with a 7-6 (7/5), 6-3 win over Chile's sixth seed Christian Garin. (AFP)

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