HARRIET DART knows she needs to improve in key moments after a nightmare first-set tiebreak in her second-round defeat against Marta Kostyuk at the US Open.

Britain’s Dart, who had already served for the opener twice, squandered five set points against the Ukrainian 19th seed – three of them on her serve.

Kostyuk had already missed three of her own in a marathon tiebreak but took her fourth opportunity and then ran away with it to seal a 7-6 (10) 6-1 victory in a match which saw the temperature hit 38°C.