US 19-year-old wildcard Frances Tiafoe beat the seventh-ranked German 4-6, 6-3, 6-4 in a second-round matchup.
US 19-year-old wildcard Frances Tiafoe beat the seventh-ranked German 4-6, 6-3, 6-4 in a second-round matchup at the year's last major tuneup for the Open, which begins August 28.
Zverev won titles at Montreal and Washington, giving him five on the year, matching Wimbledon and Australian Open champion Roger Federer for the ATP's most in 2017 after beating the 19-time Grand Slam winner in the Montreal final.
"I've beaten a lot of great players. I've beaten a lot of top-10 players. I've played a lot of big matches," Zverev said.
"I feel very confident with how I'm playing. I feel the best I ever felt going into a Grand Slam and that confidence shows on the court."
But the toll of the 10-match win streak caught up to the 20-year-old from Hamburg at Cincinnati, where he said he was only playing at 10 percent by the middle of the second set.
"I'm not a machine. I'm tired. There was no hiding it," Zverev said. "There's a physical limit to all of us and I've reached that now."
"This match is already forgotten. But it's something that's normal. It happens to all of us."
Zverev pondered withdrawing but said he made a promise to play doubles with India's Leander Paes at Cincinnati and they did, losing Tuesday in their first match.
"I'm someone who keeps my promises," Zverev said. "I actually felt OK yesterday but I only hit for 30 minutes."
As well as Zverev has played -- taking titles at Montpellier, Munich and Rome as well as his US and Canada trophies -- he is ready to put away the racquets for a while.
"I'm going to take three or four days off, play a load of golf and go to the beach and not touch a tennis racquet," Zverev said.
Zverev had his best Grand Slam run at Wimbledon, falling in the fourth round. But his back-to-back hardcourt titles have boosted him to a career-best ranking and made him a target for rivals in New York.
When he is ready and rested, Zverev counts himself among those just behind Federer and Rafael Nadal, who will return to world number one next week, as US Open favorites.
"Roger and Rafa are the biggest ones," Zverev said. "And there are a few guys coming after that. And I'm involved in that. But overall, the favorites are Roger and Rafa."