The American world number 16 reached the quarter-final of the Australian Open, the last four at the US Open and secured a first ATP title in Tokyo in his first full season on tour in 2023.
But the 21-year-old ended his campaign with first-round exits in Vienna and Paris, before another first-up loss last week in Brisbane.
"It was definitely a tough match playing against Fabian," said Shelton, who had received a first-round bye, after getting past the world number 65 in 71 minutes.
"I had a tough win against him last year at a Challenger tournament. So really cool to have a match-up first against him, he's a great player. It was a fun match but happy to get through it."
Shelton, who sent down 10 aces, faces a quarter-final on Thursday against Spaniard Roberto Carballes Baena, who beat Austrian seventh seed Sebastian Ofner 7-5, 6-3, one of several seeds to tumble out on Wednesday.
Third-seed Francisco Cerundolo of Argentina was thrashed 6-1, 6-1 by France's Alexandre Muller and eighth-seeded Australian Max Purcell lost 6-2, 6-4 to Japan's Taro Daniel.
Fourth seed Felix Auger-Aliassime of Canada lost his first singles match of the year 7-6(9), 7-5 to German Daniel Altmaier.
Second-seeded Briton Cameron Norrie survived a scare from French teenager Luca Van Assche, winning 6-3, 6-7(6), 6-1.
Norrie's quarter-final will be against Chilean qualifier Alejandro Tabilo, who got past Dutchman Botic van de Zandschulp 7-5, 6-3.
