The European 400 metres champion has made the trek to South Africa even though he knows it could affect his hopes of Commonwealth Games success next week.
"I have found it hard getting myself up for races since the Europeans," he said. "But that won't be the case here - once I get that British vest on I'll just go for it. I know it is going to be very tough to win the Commonwealths. I won't get there until the early hours of next Tuesday and will have two rounds of the 400m the next day. But maybe that won't be a bad thing. At the end of the day, I have won the title that matters. If I had lost the Europeans I would have been devastated."
Thomas's main rival on Saturday will be Jerome Young, the American who pipped him for second place behind Mark Richardson at the Grand Prix final in Moscow last weekend where both recorded the same time.
"I can beat him," said the Hampshire-based runner, who has beaten Young in their four meetings this summer. "In Moscow I went off too slow and left myself too much to do. But I have the lane inside him here which will help. If I don't win, I will be very disappointed."
The 24-year-old, who stands to collect $50,000 (pounds 31,000) for a win Britain will desperately need, added: "The track at the Johannesburg Stadium is a lucky one for me. When I trained in South Africa in 1996 I set a then Welsh record of 44.66sec there. Hopefully the altitude will help other guys in the team to perform well."