Having knocked the International Tennis Federation's number one ranked junior player Andrey Rublev (RUS) out at the semi-final stage of the Nanjing 2014 Youth Olympic Games tournament, Majchrzak upset the odds again on Saturday.
The junior world number 15 was facing Orlando Moraes Luz (BRA), the junior world number three, for the gold and won 6-4, 7-5.
“[Had I won] bronze I would be the most happy player in the world,” Majchrzak said. “But now I can’t believe what happened, really. I couldn’t believe I would come into this tournament and win it.
“I’m going to keep [this medal] very close to me all the time. Probably every evening I will come in and say, ‘Wow, I did it',” he said.
Majchrzak showed his intent early on, racing 2-0 up in the first set and holding his lead all the way through to the middle of the second.
His outstanding defence and excellent baseline play proved too much for Moraes Luz on a day when both players were soaked through in hot, humid conditions.
The Brazilian fought on and earned himself a 5-4 lead in the second set but Majchrzak regained his composure and saw the match out with some solid hitting.
Moraes Luz had been playing two, sometimes three matches a day as his mixed doubles team progressed to the quarter-finals and his doubles team to the gold medal match, and he seemed leg-weary.
Majchrzack produced some outstanding tennis to claim the gold medal
“My body’s not good today,” Moraes Luz said after the match. “I’m so tired. I don’t know how I can explain it.
“I was fighting for the gold medal for my country, but [Majchrzak] gets my congratulations."
There was another upset in the women’s singles bronze medal match as Akvile Parazinskaite (LTU) claimed her second bronze in 24 hours.
The 17-year-old defeated seventh seed Anhelina Kalinina (UKR) 6-3, 7-5, to add to the doubles bronze medal she picked up on Friday.
“I just tried to push from the first ball, to try and keep my game on the front foot, get back every single ball and fight for everything,” Parazinskaite said.
The women's doubles went to form as Iryna Shymanovich (BLR) and Anhelina Kalinina (UKR) took gold 6-4, 6-4 over Daria Kasatkina (RUS) and Anastasiya Komardina (RUS).