Del Potro stormed out of the blocks to lead Wawrinka 4 - 1 in the opening set .
Del Potro is the world No 142 and bidding to become the lowest ranked man to reach a Grand Slam semi-final in 16 years .
Del Potro , the world No 142 and bidding to become the lowest ranked man to reach a Grand Slam semi-final in 16 years , stormed out of the blocks to lead Wawrinka 4 - 1 in the opening set .