The 19-year-old swimmer outsprinted China’s Sun Yang to the wall, touching first in 1 minute, 45.14 seconds.
However, the Brit had kept something in reserve and swam a superb final 50m to re-take the lead and triumph in one minute 45.15 seconds, which broke his own British record.
“It doesn’t quite feel like a World Championships and I think that’s a good thing because I’m able to stay so chilled. I am so happy”.
Guy said: “I never thought I’d race Ryan Lochte head to head like that, he is one of my heroes”.
“Obviously I never thought I’d win the 200m as well so it’s a nice feeling”.
Guy says he has room for improvement after swimming in lane five alongside Lochte, the American who made underwater gains by kicking on his back out of turns.
“There’s more there for me though”.
Walking out, one man looked like he could have run across the water, chest pumped out, jaw squared and clenched.
“I would have loved to be on the podium but that’s the way it goes sometimes”.
World record holder Adam Peaty eased into the 100m breaststroke final, as did Ross Murdoch, whilst Ben Proud was second in his semi-final to make it into the 50m butterfly medal race.
Having won 100m Breaststroke gold last night, the World debutant sliced 0.2 seconds off his PB to win his semi in 26.42 with South Africa’s two-time champion Cameron Van Der Burgh second fastest through.
Van der Burgh won the second semifinal in 26.74, setting up Peaty and the South African for a head-to-head duel in Wednesday’s final of the non-Olympic event.
“It’s the best thing I’ve ever experienced and I just can’t wait to see my parents, ” Seebohm said.
Australia dominated the 100-meter backstroke events at the world swimming championships Tuesday, winning the men’s title and going 1-2 in the women’s final.
“Down that last length is going to be for Queen and country and I carry a lot of pride on my back that I am from Great Britain“.
It has been a strong championships for Britain in the pool, with competition running until Sunday.
Elsewhere, Chris Walker-Hebborn and Liam Tancock were fifth and eighth respectively in their 100m backstroke final, while Lauren Quigley was seventh in the women’s equivalent.