Victoria Azarenka hobbled out of the French Open after being hampered by a knee injury during her first-round match against Karin Knapp on Tuesday.

The two-time Australian Open champion had been billed as a potential challenger to Serena Williams at Roland Garros after winning three tournaments this year, but retired at 4-0 down in the deciding set on day three.

Knapp, ranked 118, took the first set 6-3 but the fifth seed showed great character to level the match by winning a tie-break 8-6 after saving a match point.

A limping Azarenka, who twice prevented the Italian from serving out the match in the second set, was given treatment but could not continue.

Azarenka had not lost since February until a back injury forced her to withdraw from the Madrid Open this month.

The Belarusian appeared to still be feeling the effects when she fell to a shock defeat against Irina-Camelia Begu at the Internazionali d'Italia, but said she was fully fit before heading to Paris only to suffer another blow.

Knapp beat world number five Azarenka in their only previous match in the same tournament nine years ago.

Australian Open champion Angelique Kerber was dumped out of the first round at Roland Garros, losing 6-2 3-6 6-3 to Kiki Bertens.