Victoria Azarenka improved her unbeaten record this year to 18-0 after battling past Germany`s Mona Barthel 6-4, 6-7, 7-6 in Friday`s second round of the Indian Wells WTA tournament.

Top seed and world number one Azarenka squandered a 5-1 lead in the second set and then came from 1-4 and 3-5 down in the third before sealing victory 8-6 in the tiebreak when her opponent hit a backhand long.

The Belarusian, who clinched her first grand slam crown at the Australian Open in January, double-faulted 10 times in an error-strewn third set and celebrated in subdued fashion when the match ended after three hours on the stadium court. She will next meet 25th-seeded Russian Svetlana Kuznetsova, who advanced after Swiss Timea Bacsinszky retired hurt with a wrist injury when trailing 3-6, 0-2.