Belarus' Victoria Azarenka retruns the ball to Italy's Karin Knapp during their women's first round match at the Roland Garros 2016 French Tennis Open in Paris on May 24, 2016. [Photo/Agencies]

PARIS - Two-times Australian Open champion Victoria Azarenka was forced to retire with a right knee injury from her opening match at Roland Garros on Tuesday when the fifth seed trailing 4-0 to world No 118 Karin Knapp in the decider.

The former French Open semifinalist got the early break to take a 2-1 lead, but it was her Italian opponent who took the first set 6-3 with big serving and hard-hitting shots.

In the second set, Azarenka took an early 2-0 lead and forced the third set with a 8-6 win in a tiebreaker, though the Belarusian seemed uncomfortable, calling for a medical timeout at 3-3 to nurse her right knee.

Azarenka battled tears throughout the third set before conceding at 0-4 to give Knapp the win.

Knapp, who made it to a career high of world No 23 last year, is thus through to the French Open women's singles last 64.

"I am really happy. I mean, I'm sorry for her. She had injury. But I think when she retired, we were having a great match," said the Italian.

Knapp next plays Latvia' s Anastasija Sevastova in the second round which she moves into for the first time since 2008.