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The 29-year-old Sydneysider who is ranked No.66 in the world, saw off the American to record the biggest win of his career after coming from a break down in the first set.

O'Connell rose to a career-high ranking of No.53 after winning the ATP Challenger event in Shanghai in September last year, but this victory over the world No.22 marks a huge step up from that.

The Australian will face another American in the Round of 32 in the form of 20-year-old rising star Martin Damm Jr, who was handed an easy path through when his opponent and fellow American Tommy Paul retired in the second set of their match, the No.11 seed taking the first set 6-4 before withdrawing when down 1-2 in the second.