Second-seeded Austrian Dominic Thiem lost to 32nd-ranked Serbian Filip Krajinovic in the round of 32 of the ongoing Western and Southern Open in New York.
Krajinovic stunned Thiem 6-2, 6-1 on Monday, losing only two service points en route to the third round of the tournament.
"I think I started really well today. I was aggressive and I'm really happy that I'm in the third round," said Krajinovic as per the ATP Tour website. "I don't think Thiem was playing today good tennis, but I took the chance and I think I deserved to win."
Krajinovic will next play Hungarian qualifier Marton Fucsovics, who beat 14th seed Grigor Dimitrov 7-5, 4-6, 6-2.
Meanwhile, world No.1 Novak Djokovic didn't have the luxury of easing back into tournament play in his first match back since the end of the ATP Tour suspension due to COVID-19 pandemic.
Djokovic had to fight it out against Lithuania's former junior World No. 1 Ricardas Berankis and he eventually triumphed 7-6(2), 6-4 to improve his perfect record in 2020 to 19-0.
The Serb, who became the first singles player in history (since 1990) to capture all nine ATP Masters 1000 titles when he defeated Federer in the 2018 Cincinnati final, has advanced to the third round where he will face American Tennys Sandgren.