Wozniacki endured a painful Tuesday evening in Luxembourg
Top seed Caroline Wozniacki was forced to retire from the Luxembourg Open with a hamstring and back injury when she led local star Anne Kremer 7-5 5-0.
The Danish world number six was set to face Melanie Oudin in the first round but the American withdrew with illness.
US Open champion Kim Clijsters started her campaign with a 6-2 6-2 win over American Meghann Shaughnessy.
Jelena Jankovic avenged her US Open loss to Kazakhstan's Yaroslava Shvedova to progress in the Kremlin Cup.
Jankovic, defending champion and second seed, is fighting for a place at next week's season-ending WTA Championships and dug deep to secure a 6-4 5-7 6-1 victory to make the second round in Moscow.
"Today I knew what to expect from her," said the Serb, who lost to Shvedova in the second round in New York.
She will next face Lucie Safarova of the Czech Republic, while top seed and home favourite Vera Zvonareva also eased through the opening round with a comfortable 6-2 6-2 win over Romania's Ioana Raluca Olaru.
Wozniacki was last year's Luxembourg runner-up to Russian star Elena Dementieva and the Dane, who lost to Clijsters in the US Open final, had been strolling to a routine win before injury struck.
She was cruising past Luxembourg's 34-year-old Kremer when she pulled up with a problem with her left hamstring and her back.
Another seed bowing out on Tuesday was seventh seed Kateryna Bondarenko of Ukraine who succumbed 6-0 6-2 to Italian Roberta Vinci.
Nikolay Davydenko, fresh from winning the Shanghai Masters on Sunday, is the leading name in the men's draw and is scheduled to face fellow Russian Marat Safin in the first round on Wednesday.
The three-time Kremlin Cup champion is still trying to secure his place in the eight-man field for the ATP World Tour Finals in London next month.