Kei Nishikori exited in the quarterfinals of the ABN AMRO World Tennis Tournament in Rotterdam on Friday, losing a tight two-setter against Croatia's Borna Coric 7-6 (7-2), 7-6 (7-4).
World No. 45 Nishikori, who missed most of the 2020 season following major elbow surgery, blamed a lack of concentration for his loss of two tiebreakers against the 26th-ranked Coric.
"I'm disappointed because I had the chance to win (but) I made a lot of mistakes," Nishikori said. "I tensed up because I started thinking too much about wanting to win."
The former world No. 4 is trying to work his way back to the top 10 following the elbow operation in late 2019. A positive coronavirus test delayed his return to the tour last August before a shoulder injury suffered at the French Open last October sidelined him for the rest of the year.
His opening-round match in Rotterdam, a straight-sets win over Canadian seventh seed Felix Auger-Aliassime, was his first ATP Tour victory in five months.
The 31-year-old won his next match against Australian Alex de Minaur to advance to the quarterfinals.
"There were ups and downs, but I gained a lot of confidence this week," Nishikori said.