WASHINGTON (Kyodo) -- Former world No. 4 Kei Nishikori advanced to the third round of the Citi Open on Wednesday following a straight-sets win over American qualifier Donald Young.
After receiving a first-round bye in the men's singles, the seventh-seeded Nishikori, one of four past champions in the men's field, slammed eight aces while converting three of five break points to take down Young 6-3, 6-4 at Rock Creek Park Tennis Center.
The 28-year-old Japanese, currently ranked 20th, remains undefeated in seven match-ups with the No. 234 Young.
"It's been a while since I played on a hard court and I was nervous, but I played good tennis from the beginning and I was able to focus until the end," said Nishikori.
"I had more service aces than I normally do and I'm happy about that," he said.
Nishikori is making his fifth appearance at the hardcourt Citi Open -- one of 13 ATP World Tour 500 series tournaments in the season -- since he crashed out in the third round of his 2013 debut.
He defeated American John Isner in 2015 to claim the championship, but did not participate the following year. He made a semifinal exit last year.
On Thursday, Nishikori will face No. 26 Denis Shapovalov. The 19-year-old Canadian handed a first-round loss to Nishikori at the Mexican Open earlier this year in their only other meeting to date.
In the men's doubles, Japan's Ben McLachlan and Croatia's Ivan Dodig beat Brazil's Marcelo Melo and Poland's Lukasz Kubot 6-4, 7-5 to advance to the quarterfinals. They face American pair Frances Tiafoe and Denis Kudla on Thursday.
Japan's Shuko Aoyama and Czech partner Renata Voracova, who won the women's doubles last year, were knocked out in the quarterfinals by Americans Kristie Ahn and Lauren Davis, losing 5-7, 6-4, 11-9.