Olympic champion Andy Murray picked up where he left off in Rio by beating Juan Monaco in the Cincinnati Masters.
Murray, the first player to defend an Olympic singles title, secured his 19th successive victory by a 6-3 6-2 margin.
The 29-year-old, who had a bye in round one, will play South African Kevin Anderson in the third round.
Third seed Rafael Nadal, who won the men's doubles in Rio, beat Uruguay's Pablo Cuevas 6-1 7-6 and next faces Borna Coric who defeated Nick Kyrgios.
The Australian smashed three racquets in one changeover during his 7-6 (7-2) 4-6 7-6 (8-6) defeat by Croatia's Coric.