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Italian qualifier Lorenzo Musetti may be ranked 120th in the world but he claimed his maiden top-10 scalp on Tuesday, beating Diego Schwartzman in the first round of the Mexican Open.
Musetti, the Australian Open boys' singles champion in 2019, secured a 6-3 2-6 6-4 win over ninth-ranked Schwartzman in a stunning success.
"There is a lot of work, a lot of sacrifice and I think some tears left in my eyes," the 19-year-old said during his on-court interview.
"I'm really proud of myself, but now I'm going to work harder and focus on the next days."
Musetti, whose forehand was excellent, defeated both Stan Wawrinka and Kei Nishikori at the Internazionali d'Italia in Rome last year.
Top seed Stefanos Tsitsipas had no such trouble, easing into the second round, where he will face big-serving American John Isner.
The Greek world number five beat France's Benoit Paire 6-3 6-1, aided by a strong display on serve.
"I didn't know what to expect from Benoit today," Tsitsipas said. "I think it's kind of tricky playing a tournament you haven't played before."
On facing 27th-ranked Isner next up, Tsitsipas added: "He's a guy that serves really well so that's going to be the biggest element and the one thing that I'll really have to resolve during that match."
Fourth seed Milos Raonic sent down 13 aces as he defeated American Tommy Paul 7-6 (8-6) 6-4.
In the late match, fifth seed Grigor Dimitrov got past Adrian Mannarino, who retired when trailing 6-4 3-0.