Chinese teenager Shang Juncheng defeated Ben Shelton for the second time in two weeks on Tuesday, downing the 14th-seeded American 6-2, 3-6, 6-3 to reach the third round of the ATP hard court tournament in Washington.

"I'm definitely very happy right now," the 18-year-old Shang told ATPTour.com. "Ben is not an easy player to beat. I think I've got a game that kind of suits me to playing him."

Shang, who ousted Shelton 6-4, 6-4 at the Atlanta Open last week, converted five of his nine break point chances on Tuesday and polished off the victory in one hour and 42 minutes.

"In the past two weeks, I've been returning really well," Shang said. "So I think that was the key today, to break a few service games from him."

The left-hander from Beijing posted his fourth victory in as many days, including two qualifying victories and a first-round triumph over Emlio Gomez.

He had started his year by qualifying for the Australian Open and winning a first-round match but was then slowed by mononucleosis.

Once back on court he qualified for the French Open, but didn't make it out of the first round.

Shang's reward for Tuesday's win will be a meeting with American second seed Frances Tiafoe, who opened his campaign with a 7-6 (7/5), 7-6 (7/5) win over Russia's Aslan Karatsev.

"Unbelievable match tonight. It was so tight all the way through, he really made me earn it," Tiafoe said afterward.

"I'm just happy to get through. He's a tough player, he's made the semis of a Grand Slam and he's very, very dangerous. Very tough first round."

Elsewhere Tuesday, former world number four Kei Nishikori of Japan withdrew with a left knee injury before his scheduled first-round match against South Africa Lloyd Harris.

Nishikori, 33, returned to competition from a 20-month injury absence in June, winning a low-tier Challenger Tour title in Puerto Rico before reaching the quarter-finals last week in Atlanta in his first ATP tournament since 2021.

Nishikori said he had experienced knee pain in Atlanta and after testing the knee Tuesday decided that he was "not good enough to play."