Coco Gauff has reached the semi-finals of a WTA Tour event for the first time as her run continued at the Linz Open. The American, 15, beat top seed and world number eight Kiki Bertens, of the Netherlands, 7-6 (7-1) 6-4 in Austria. It was Gauff’s first win over a top-10 player, at an event in which she only came into the draw as a lucky loser. “I’m really happy and proud of myself,” said Gauff, who will face Germany’s Andrea Petkovic in the semi-finals on Saturday. She becomes the youngest player to reach a semi-final on the WTA Tour since the Czech Republic’s Nicole Vaidisova won in Tashkent, Uzbekistan, in 2004. After a tight first set, Gauff came through the tie-break convincingly. She then grabbed the only break of serve at 1-1 in the second set and held her nerve for a memorable win. It continues the breakthrough year in Gauff’s fledgling career after she reached the last 16 at Wimbledon, which included a win over five-time former champion Venus Williams, and got to the third round at the US Open.