Melbourne - Jack Sock completed a bleak opening day for the American contingent at the Australian Open when he was knocked out by Japan's Yuichi Sugita on Monday.
The eighth seed tumbled out of the tournament on the end of a 6-1, 7-6 (7/4), 5-7, 6-3 loss in two-and-a-half hours to the 41st-ranked Sugita.
It capped a dark day for the American contenders with Venus Williams and US Open champion Sloane Stephens knocked out of the women's event along with CoCo Vandeweghe and men's 16th seed John Isner.
Sock had a troubled build-up the year's opening Grand Slam after a hip injury during his Hopman Cup match with Sugita in Perth earlier this month.
He also bowed out in the Auckland Classic second round to German Peter Gojowczyk, compounding his poor preparations.
Sock made a total of 52 unforced errors and dropped his serve seven times to Sugita and was always chasing the match after losing the opening two sets.
The Japanese star next faces Croatia's Ivo Karlovic.