Dane Wozniacki came out of retirement last year after the birth of her two children and reached the fourth round of the US Open where she lost to eventual champion Coco Gauff (19) but showed she could still compete at the highest level.
The 2018 Melbourne Park champion was up 5-2 in the opening set when 20th seed Linette took a medical timeout after struggling to move, and the ailing Pole was unable to continue shortly after going down by two games in the second set.
"It's never nice to see that. Magda is a good friend of mine. I followed her last year and how unbelievably she played last year here," said Wozniacki, who will next play Maria Timofeeva (20) after the Russian beat France's Alize Cornet (33) 6-2 6-4.
"Obviously this isn't how I wanted to finish it. I really hope it's not too serious and she recovers soon."
