Kei Nishikori overcame Pierre-Hugues Herbert 3-6, 6-1, 6-4 at the Rotterdam Open on Tuesday on his return from the knee injury which forced him to retire during his Australian Open quarterfinal against Novak Djokovic.
"It wasn't easy, as he [Herbert] was playing good tennis and serving well," Nishikori said.
"I had a little bit of trouble with my return game and couldn't convert the important points. I just tried to play steady, as I wasn't playing bad, and I tried to stay aggressive."
"It's going to be tough, particularly as our last match was four tiebreaks, across four hours, in Melbourne," the Canadian said.
Elsewhere, No. 2 seed Karen Khachanov fell to a surprise defeat against home favourite Tallon Griekspoor. The Dutchman -- ranked No. 211 in the world -- defeated the highly rated Russian 3-6, 6-3, 6-2.
Canada's Denis Shapovalov made it through with a comfortable 7-5, 6-3 win over Croatian qualifier Franko Skugor. He will face Tomas Berdych in the second round after the Czech battled past qualifier Gilles Simon, winning 7-6 (8), 6-4.