Melbourne - Roger Federer has faced most top players on tour but never his Australian Open quarter-final opponent Tennys Sandgren, and he is wary of the American as he attempts to set up a potential last-four clash against Novak Djokovic.
The Swiss great swept into the last eight for a record 15th time at Melbourne Park after a first-set wobble against unheralded Hungarian Martin Fucsovics.
He now goes toe-to-toe on Tuesday with the last American in the men's draw who has belied his 100 world ranking by beating eighth seed Matteo Berrettini and 12th seed Fabio Fognini.
The Australian Open is Sandgren's favourite Major and he came from nowhere to make the quarter-finals in 2018, eventually losing to South Korea's Chung Hyeon.
Federer has never played the 28-year-old, and is not underestimating the challenge as he looks to win his first Grand Slam since Melbourne in 2018 and seventh in Australia.
"I wonder why he's not ranked higher, to be honest. Every time I see him play, I feel like he plays very well. He's got a lot of stuff in his game that he's deserving of being higher," said the world number three.
"I didn't follow him that closely. I mean, I remember he played Chung in the quarters the last time I played the semis, to play Chung with his blisters and all that.
"I remember watching that closely because I was going to maybe play the winner."
Federer, who is chasing a 21st Grand Slam title, was also watching as Sandgren tamed firebrand Italian Fognini on Sunday.
"It was impressive how he was competing. I'm looking forward to a tough one."
The odds are heavily stacked in the 38-year-old's favour. Of the 14 Australian Open quarter-finals he has made, Federer has won them all.
Sandgren said he was just happy to have another chance at a big tournament.
"Getting to play in a big stadium, getting to play in front of a lot of people, because I've played a lot of tennis in front of the very few people, the fact I get to do that seems to bring out the best tennis in me," he said.
"It seems if I play pretty well, I have a shot."
At stake is a semi-final against either second seed Djokovic or Canadian Milos Raonic.
The Serbian has been in fine touch as he targets a record eighth title and a 17th Slam crown, but he has his work cut out against one of the biggest servers in the game.
So far Raonic, who has failed to beat Djokovic in nine previous attempts, has played 59 service games and is yet to be broken, pumping down 82 aces.
"Obviously it's a huge advantage when you hit serves from that height. You can hit any angle, anything you really want," said Djokovic. "That puts a lot of pressure on your opponent."
He said fellow big servers John Isner and Ivo Karlovic perhaps had the edge on Raonic, but the Canadian was a better mover around court.
"If the returner gets the ball back in play, then I think Raonic is better than these two guys," he said. "But I feel like maybe you could read his serve better than Isner and Karlovic."
Raonic has been plagued by injuries throughout his career, but has found success in Melbourne, reaching the semi-finals in 2016 and the quarters three other times, including last year.
"I think we play quite opposite from each other, and he's done a good job in the past neutralising my serve," said the 32nd seed of Djokovic.
"So I have really got to focus on my things well and be the one dictating."