Angelique Kerber emerged victorious from a gruelling two-and-a-half-hour showdown with Agnieszka Radwanska to reach the semi-finals of the Bank of the West Classic.
There were twists and turns aplenty in Stanford but in the end it was German fifth seed Kerber - last year's runner-up - who preserved her energy to prevail 4-6 6-4 6-4.
She told wtatennis.com: "After the first set I was just fighting and playing point by point.
Addressing the centre-court crowd, she added: "You were all so amazing and I didn't want to disappoint you - I knew I just had to keep fighting!"
Kerber will next tackle eighth seed Elina Svitolina after the Ukrainian number one rallied from behind to beat Alison Riske 4-6 7-5 6-1.
The other last-four tie will see Varvara Lepchenko meet fourth seed Karolina Pliskova.
The former player recovered from an early break down to edge Mona Barthel 6-7 (3/7) 6-2 6-3 while Pliskova eliminated Ajla Tomljanovic with a more straightforward 6-2 6-4 win.
Source: PA
