Four months ago, Ryan Harrison was lying in a hospital bed after undergoing surgery to repair a torn right extensor tendon. On Monday, the American showed he’s back in business by easing past Damir Dzumhur of Bosnia-Herzegovina 6-3, 6-2 at the Delray Beach Open by VITACOST.com.

“I haven’t played a match at this level in quite a long time. Today was a good sign that it's coming together,” Harrison said. “I’m only 27 and most guys are peaking in their early to mid-30s, so I’ve got a long way to go… I feel like I’ve got a lot of good tennis left in me.”

The win is Harrison’s first tour-level victory since last April in Houston. Awaiting him in the next round is rising South Korean Soonwoo Kwon, who upset seventh-seeded Frenchman Adrian Mannarino 1-6, 6-3, 6-2.

#NextGenATP American Brandon Nakashima prevailed in his ATP Tour debut with a 7-6(4), 6-1 win over Czech Jiri Vesely. The 18-year-old took the last six games against the recent Pune champion to advance in one hour and 22 minutes. Nakashima will play the winner of third-seeded American Taylor Fritz and British qualifier Cameron Norrie.

“It was really fun being out there. It was a great match, so I’m really excited to keep going here and hopefully keep doing well,” Nakashima said. “I always knew that my game can compete with these guys, so I had to stay focussed and keep competing for every point.”

American Mackenzie McDonald also completed a return from injury by taking out Japanese Yasutaka Uchiyama 6-2, 7-6(5). McDonald missed the last six months of the 2019 season after having surgery to repair a proximal hamstring tendon. He returned to action in January and has now earned his first tour-level win in 10 months.

Monday’s night session saw Japanese Yoshihito Nishioka score the upset of the day by rallying to defeat fifth-seeded Aussie John Millman 3-6, 6-4, 6-2. Nishioka improved to 6-3 this season as he looks to reach his first quarter-final of the year.

What a comeback. 🙌
What a way to win it. 🤯@yoshihitotennis rallies to defeat John Millman in 3 sets at the @DelrayBeachOpen. 👏

🎥 : @TennisTV pic.twitter.com/3nBLFnJPqT

— ATP Tour (@atptour) February 18, 2020

"I had to move a lot and focus a lot, so I'm really happy for the win and feeling very confident," Nishioka said. "I want to go as far as I can here."

American Frances Tiafoe, the 2018 champion, closed out Monday's schedule with a 7-6(3), 6-2 victory over Ecuadorian qualifier Emilio Gomez. Tiafoe, who picked up his first tour-level win of the season, will face the winner of top-seeded Aussie Nick Kyrgios and American Tommy Paul.