Ohio, Aug 17 (IANS): American hopeful Frances Tiafoe upset Alexander Zverev 4-6, 6-3, 6-4 at the Western and Southern Open in Cincinnati here, handing the German rising star his first loss of the North American summer hard-court tennis season.

Unlike at tournaments in Washington and Montreal, where Zverev barely survived his opening match before cruising to the title, the fourth-seeded German was on Wednesday unable to get past the first hurdle at this US Open tune-up event, reports Efe.

The 19-year-old Tiafoe showed off his strong return game in the second-round match to break his opponent's serve on six occasions, including three times in the second set when he turned the contest in his favour.

Zverev defeated Swiss great Roger Federer to win the Rogers Cup in Montreal on Sunday. But after the victory he said he was feeling fatigued and had low expectations for himself in Cincinnati, which contributed to his troubles with a poor serving day here.

For the match, the 20-year-old German struck 12 aces but also double-faulted a whopping seven times.

The 87th-ranked American wildcard, meanwhile, demonstrated his power on serve by throwing down eight aces.

Next up for Tiafoe in Thursday's round of 16 will be fellow American John Isner, who fired 16 aces in his second-round victory Tuesday over countryman Tommy Paul.

The world's best players are getting their games in gear in Cincinnati ahead of the fourth and final Grand Slam event on the tennis calendar, the US Open, which gets under way on August 28.