Shelton expects to face a full-strength Alcaraz despite the fact that the seven-time Grand Slam champion is playing his first tournament in more than four months after recovering from a right wrist injury.
"To me he looks very normal - his normal, I mean," Shelton said.
Shelton is among a strong contingent of American men out to end their 23-year Grand Slam title drought. At least one will reach the semi-finals as 11th-seeded Frances Tiafoe squares up against fellow American Alex Michelsen on Tuesday, when women's play sees two-time defending champion Aryna Sabalenka vying for a semi-final spot against Wimbledon champion Linda Noskova.
AFP Sport looks at Tuesday's action at the US Open:
Alcaraz leads head-to-head 3-0
Alcaraz, an unknown quantity coming into his title defence after his injury layoff, has improved with every match, and he'll likely need to do so again against Shelton.
Shelton came through three tough four-setters before blowing past Stefanos Tsitsipas behind a stellar performance on serve and said he still has room to improve against Alcaraz.
"It wasn't my AA tennis," said Shelton, who said it would take "special tennis" to topple Alcaraz.
"I think he's playing extremely high level," Shelton said.
With fitness fears allayed and his powerful forehand at full strength, Alcaraz said his serve is the shot that is still not quite there. He has effectively applied pressure by mixing up his return game - will that be enough to unsettle Shelton and derail the former semi-finalist's bid for a home Grand Slam?
Frances Tiafoe vs Alex Michelsen
Tiafoe leads head-to-head 2-0
Tiafoe pulled out all the stops in a straight-sets victory over former champion Daniil Medvedev that sent the American into the US Open quarters for the fourth time.
A two-time US Open semi-finalist, Tiafoe rallied from 5-6 down in the third-set tiebreaker to snatch the victory that left him "super-pumped".
He'll bring that energy against 22-year-old compatriot Michelsen, who hasn't dropped a set in reaching the first Grand Slam quarter-final of his career.
"He's young, he's going to be juiced up," Tiafoe said. "Let the best man win."
Aryna Sabalenka vs Linda Noskova
Sabalenka leads head-to-head 2-0
Noskova put herself among the game's elite with her maiden Grand Slam title at Wimbledon, but two-time defending champion Sabalenka will require the best the 21-year-old Czech has to offer.
Sabalenka, whose indifferent mid-season results mean she could lose her world number one ranking even if she completes a rare Flushing Meadows three-peat, hasn't dropped a set in reaching the quarters for a sixth time.
"It's going to be some fast tennis," Noskova said, noting the many weapons Sabalenka has at her disposal.
"She's very consistent in whatever she does. Either the serve or she's going into every return that she can, just playing so, so solid from the baseline and not being afraid to go on the net as well and using some drop shots or slices to mix up the rhythm."
Jessica Pegula vs Emma Navarro
Pegula leads head-to-head 4-0
World number three Pegula holds the upper hand heading into the first all-American US Open women's quarter-final since Serena Williams defeated sister Venus in 2015.
Pegula's four career victories over Navarro include a win at Cincinnati last month.
"Jess is really tough," Navarro said. "She probably hits the flattest ball on tour, keeps it low, can hit it short and flat or deep and it looks the same.
"I have lost to her a few times now, but there's always that time when you get over the hump and get the win."
Pegula has won two titles this year and is among the players with a chance to supplant Sabalenka atop the world rankings in New York. The 32-year-old will have to go all the way to a first major title to have a shot at number one.
Navarro, a former world number eight, has come on strong after an extended break to focus on health issues, winning her first title in more than a year at Strasbourg in May.
