The Australian Open champion, who now faces Poland's Agnieszka Radwanska, grabbed a hard-fought first set but had to fight off a sustained challenge from Barthel, who had eliminated former world number one Ana Ivanovic and world number seven Mario Bartoli of France in the previous rounds.

"She showed some great tennis today," Azarenka said in a courtside interview. "I always said since the start of the year she is the one to watch." The Belarussian has lost only once this season after going 26 matches without defeat.

"I am trying to become better and better. This is new for me (being No.1) but I feel good and confident being there." The pair traded punishing blows throughout the second set with Barthel twice moving to within two points of winning the set, at 5-4 and 6-5, before Azarenka briefly pulled herself out of the hole.

Fired on by the local crowd who mere mesmerized by her mix of raw power and intelligent play, big-serving world No. 35 Barthel, who won her maiden WTA tour event in Hobart in January, grabbed the second set on her first set point.

Another huge backhand winner down the line gave her the first break opportunity at the start of the second set and Azarenka cracked under pressure to send the next point long.

But she recovered to twice break back and, after squandering a match point at 5-4, finished her opponent off two games later.

Earlier Radwanska had to come from a set down to beat China's Li Na 3-6, 6-2, 6-3.

Later on Friday world No. 2 Maria Sharapova takes on Australian Sam Stosur while another German, Angelique Kerber battles it out with Wimbledon champion Petra Kvitova for a spot in the last four.