Kei Nishikori defeated three-time champion Juan Martin del Potro of Argentina in straight sets Thursday to reach the quarterfinals at the rain-delayed Citi Open.
Nishikori, winner of this tournament in 2015, prevailed 6-4, 7-5 in 1 hour, 49 minutes at the Rock Creek Park Tennis Center in Washington.
"I think I was able to win in two sets because I made fewer mistakes than in my first match (in the second round). Overall my serve was solid," Nishikori said.
"It's been pretty tough," Nishikori said, referring to finishing his match in the early hours of the morning again because of the rain delay.
World No. 9 Nishikori, whose victory was only his second in seven meetings against 32nd-ranked del Potro, will face American wild card Tommy Paul in the quarterfinals.
The Citi Open is considered a warm-up event for the U.S. Open, which kicks off later this month.