Number one seed Kevin Anderson was forced to retire after the first set of his opening round match with Austin Krajicek at the Delray Beach Open in Florida.
The South African, who has been suffering with a knee complaint in recent weeks, felt pain in his shoulder and called time on the game after losing the first set to a tie-break.
Juan Martin del Potro enjoyed a day to remember as he won his first game in 11 months.
The Argentinian, the 2009 US Open champion, took to the court for the first time since last March following a third operation on his wrist and only needed 61 minutes to brush aside American Denis Kudla 6-1 6-4.
Second-seed Bernard Tomic followed Anderson out of the tournament after a straight-sets defeat to American Rajeev Ram.
Ram won 7-6 6-1 on the hard court, with the Australian Tomic struggling to adapt having come from a clay tournament in Ecuador a fortnight ago.
America got the better of Australia in two other ties as well as Noah Rubin got the better of big-serving Sam Groth 6-4 4-6 7-6 while Steve Johnson beat John Millman 6-3 5-7 7-6.
There were further American winners in Donald Young, who was a set and a break up when his opponent Mikhail Kukushkin retired, Sam Querrey - a 4-6 6-3 6-4 winner against Thimeo De Bakker - and Tim Smyczek, who beat compatriot Taylor Fritz 6-3 6-3.
Fifth-seed Jeremy Chardy made light work of Matthew Ebden, earning a 6-2 6-1 win, while Japanese qualifier Tatsuma Ito beat Dennis Novikov 6-2 6-4.
Benjamin Becker, Damir Dzumhur and Illya Marchenko were all straight-set winners over Radu Albot, Ricardas Berankis and Marcel Granollers respectively.
Source: PA
