'I'll have to play well to beat him.'

At the bottom of the draw, second seed Mardy Fish made a precautionary pullout with an existing abdominal strain to hand Argentine Leonardo Mayer a walkover to join another untried semi-finalist, Carsten Ball.

Australian-American Ball defeated John Isner 6-4, 6-4 with the American favouring a previously-injured right ankle but insisting it was not the cause of his downfall.

Both the 205th-ranked Ball and Mayer, former junior rivals, are into uncharted territory in their first-ever ATP semis.

Ball has never before even competed in a main draw but is trying to keep his head in potentially overwhelming circumstances. 'A bit of confidence goes a long way, I was not noticing Isner today and didn't know if he was injured.'

Fish had originally suffered his injury during a Davis Cup loss to Croatia nearly a month ago.

'It only bothers me on serve but I don't want to take any chances. To be healthy is the most important thing. It's a day-to-day thing and I decided to pull out after treatment late last night.'

Mayer is hoping to duplicate the feat of Argentine compatriot Juan Del Potro, who won the tournament last year. 'I'm two matches away from keeping the LA trophy in home soil.

'I'm surprised to reach my first career semi-final with a withdrawal. I was ready to play today, expecting a tough match against Mardy who is a great player.'