Kei Nishikori marked his ATP World Tour return with a win at the New York Open, while Steve Johnson suffered a shock loss on Tuesday.
Nishikori missed the end of last year due to a wrist injury, but the Japanese star claimed a Challenger Tour title in Dallas earlier this month after not playing the Australian Open.
In his first ATP World Tour appearance since last year's Canadian Open, fifth seed Nishikori beat Noah Rubin 7-5 6-3 in the opening round in New York.
Awaiting the 2014 US Open runner-up in the second round is Evgeny Donskoy, who claimed a 7-5 6-1 win over Victor Estrella Burgos.
The only other seed in action on Tuesday was Johnson.
But the American suffered a surprise defeat at the hands of Adrian Menendez-Maceiras, the qualifier prevailing 1-6 6-3 7-6 (9-7).
Menendez-Maceiras will next meet Jeremy Chardy after the Frenchman edged Stefano Travaglia 4-6 7-6 (7-1) 6-4.
Meanwhile, Ernesto Escobedo overcame Mackenzie McDonald 4-6 6-3 7-6 (7-5), Radu Albot beat Bjorn Fratangelo 6-4 6-3, Peter Gojowczyk brushed past Blaz Kavcic 6-4 6-2 and Mikhail Youzhny got past Matthew Ebden 3-6 6-3 6-4.