To say Mackin is not expected to win, barring an injury befalling his illustrious Swiss opponent, is the biggest understatement in sport for years.
The 24-year-old Scot has never won a match on the ATP Tour and is ranked 262 in the world on the strength of his results in lower level Challenger events. He is about to be sacrificed by Britain's captain, Jeremy Bates, to save Greg Rusedski's legs in the hope the tactic produces a place in the World Group next year.
At best, Mackin will make Federer work for his points on the indoor clay court at the Palexpo, and perhaps win a few games. That was the story of Mackin's only previous appearance in the Davis Cup, when he was out-classed by Mark Philippoussis, of Australia, 6-3, 6-3, 6-3, in the first round of the World Group in Sydney in 1999.
The only way Bates's strategy can work is if Mackin's compatriot, the 18-year-old Andy Murray, defeats the Swiss No 2, Stanislas Wawrinka, in today's second singles, then partners Rusedski to success against Federer and Yves Allegro in tomorrow's doubles, and Rusedski beats Wawrinka in the concluding singles on Sunday.
Murray is expected to meet Federer for the first time in singles in the opening match on Sunday, an experience that should serve the British No 2 well in the long term, if not on the day.
He has already demonstrated his potential this year. He partnered David Sherwood to a doubles victory in Israel on their Davis Cup debut in March. He reached the third round at Queen's and at Wimbledon, won two Challenger tournaments in America, and won a round at the US Open as a qualifier.
Bates, with the odds stacked against him from the moment the draw was made and Federer declared his intention to play after resting as Switzerland lost to the Netherlands at home in March, decided to gamble with his team selection.
"Playing against Federer is very difficult," Bates said. "So we looked at the tie realistically to find a way of taking the doubles and two singles against their No 2 and fighting for anything we can get from Roger as well.
"I've told Alan, 'you have to play up to the level of somebody like that and give absolutely everything, a lot of heart and a lot of aggression, more than you're used to'. We have to have somebody in a good state of mind, willing to have a go."
Mackin, who has played 15 tournaments at various levels this year and lost in the first round 13 times, hopes the experience of playing against Philippoussis in Sydney will strengthen his resolve.
"Federer is definitely a difficult opponent to play against," Mackin said, "but obviously I'm just happy to have the chance to go out there and play. I've nothing to lose. Roger has only been beaten a handful of times this year. I can only do my best and see what happens."
Clay is not Federer's favourite surface, but he did reach the semi-finals at the French Open before losing to the 19-year-old Spaniard Rafael Nadal, who won the tournament. Mackin uses a training base in Majorca, and has hit there with Nadal and Carlos Moya, the former world No 1.
Bates believes his selection has transferred the pressure on to the Swiss, particularly Wawrinka, who is ranked 60th in the world - 50 places above Murray - but has lost all three of his Davis Cup matches. Not that Bates underestimates the 20-year-old Wawrinka, saying: "You don't get where he is without having talent."
Murray said he would not have minded playing Federer in the opening match. "We've looked at the best way to win the tie. We've got to take two points off Wawrinka, whether I play on first day or the last day."
Wawrinka - Stan to his team-mates and to the cowbell-ringing 7,000 Swiss supporters expected in the hall today - has not exactly taken a shine to Murray, telling the Geneva Tribune that he thought the Scot took himself too seriously, on and off the court.
Federer said he was surprised by Bates's tactics. "I thought he might leave Murray out on the first day," he said. "I'm pleased I don't have to play Murray on the first day, and Stan is pleased he doesn't have to play Greg on the first day," he said.
As for home expectations that he would win three best-of-five sets matches in three days, Federer said: "It's not the easiest thing. It's a bit like that when I come to a tournament as the No 1 seed. But I want to win most, so the pressure comes from me."
Different level: Roger Federer and Alan Mackin head-to-head standings
Born: Basle, Switzerland, 8 August 1981.
Lives: Oberwil, Switzerland.
Height: 6ft 1in.
Turned pro: 1998.
World ranking: No 1 (for 87 weeks)
Grand Slam titles: 6.
Total singles titles: 32.
Doubles titles: 7.
2005 singles record: 71-3.
2005 singles titles: 10.
Davis Cup record: 26-10.
Career prize-money: $19m (£10.6m).
Born: Paisley, Scotland, 11 August 1981.
Height: 5ft 8.5
Turned pro: 1997.
World ranking: 262
Grand Slam titles: 0.
Total ATP titles: 0.
Career ATP record: 0-10.
ATP doubles titles: 0.
2005 singles record: 0-6.
2005 doubles record: 0-1.
Davis Cup record: 0-1.
Career prize-money: $118,565 (£66,159).
ORDER OF PLAY
Draw for Switzerland v Great Britain, Davis Cup World Group play-off: (Geneva Palexpo)
Today (12pm BST): Roger Federer (Swit) v Alan Mackin (GB); Stanislas Wawrinka (Swit) v Andy Murray (GB).
Tomorrow (12pm): Federer and Yves Allegro v Greg Rusedski and Murray.
Sunday (11am): Federer v Murray; Wawrinka v Murray.