Defending champion Sam Querrey easily advanced to the quarterfinals of the Delray Beach Open with a straight set 6-2, 6-3 victory Thursday over fellow American and doubles partner Jared Donaldson.
"I was just swinging freely through everything and did a good job of committing to it," the fourth-seeded Querrey said. "Even if I'm missing, I'm trying to keep doing it if it's the right play. That's what I've been working on a lot the past couple of days."
Querrey recorded 13 aces and didn't lose a serve while breaking Donaldson three times.
Next up for Querrey will be No. 7 seed Juan Martin del Potro of Argentina, who ousted Damir Dzumhur of Bosnia and Herzegovina 7-6 (5), 4-6, 6-3.
Young, ironically lost in the Del Ray final in 2015 to Karlovic, staved off 13 of 15 break points to defeat the Fritz, ranked 112th in the world.
"It's all a bit of a blur right now. I was just trying to compete, to fight, and I was able to win. I don't know how," Young told reporters. "It's tough playing these young guys, it brings a different pressure. They want to beat you bad, you don't want to lose to them. I've been there before, the older guys didn't let me in, so I'm trying to hold them off for as long as possible."
Young, ranked 69th coming into the tournament, was supposed to face Belgian Steve Darics in one of Friday's quarterfinal matches. But Darcis, who advanced earlier Thursday with a 6-3, 7-5 win over Georgia' Nikoloz Basilashvili, was forced to withdraw, sending Young into the semifinals on Saturday.
Darcis tweeted he had to withdraw for family reasons and wished Young good luck.