US Open champion Coco Gauff overcame huge resistance from the unseeded Marta Kostyuk on Tuesday to move into her first Australian Open semi-final. The American fourth seed wore down the Ukrainian in hot conditions on Rod Laver Arena to clinch a scrappy encounter 7-6 (8/6), 6-7 (3/7), 6-2 in three hours and eight minutes. She will face either defending champion and second seed Aryna Sabalenka or ninth-seeded Barbora Krejcikova on Thursday for a place in the final. After showing ominous form to race into the last eight, it was a far from convincing display by the 19-year-old, who struggled on serve and fluffed countless opportunities.