SHENZHEN          -       Ashleigh Barty is already guaranteed to finish the year as world No 1 and the 23-year-old Australian served notice here on Thursday of her aim to finish the season in appropriate fashion at the Shiseido WTA Finals.  Barty secured her place in Saturday’s semi-finals with her best performance of the tournament so far when she beat Petra Kvitova 6-4, 6-2 in less than an hour and a half. After losing her second round-robin match to Kiki Bertens, Barty knew that she had to beat Kvitova to guarantee her progress in the competition. She did so in emphatic style. The French Open champion played superbly on most of the big points. The slow court did not suit Kvitova’s big-hitting game and Barty returned serve particularly well.  The statistics of the first set told their own story. Kvitova, going for her shots, had four breakpoints, but Barty successfully defended all of them. Kvitova had to defend only three breakpoints, but Barty converted the last of them at the end of the eighth game to take a decisive 5-3 lead. The world No 1 drove home her advantage in the second set to secure her place in the last four.  Belinda Bencic became the second player to qualify for the semi-finals from the Red Group after Kiki Bertens retired early in the second set when trailing the Swiss 7-5, 1-0. Bertens, who had come into the tournament as an alternate after Naomi Osaka withdrew after her first match because of a shoulder injury, served for the opening set at 5-4 but was feeling unwell and quit after the tournament doctor came on court to examine her.  With Elina Svitolina having already qualified for the semi-finals from the Purple Group, the last berth will be filled by the winner of Friday’s meeting between Simona Halep and Karolina Pliskova in the concluding round-robin match.