Felix, runner-up in Athens and Beijing to Jamaica's Veronica Campbell-Brown, overcame a sluggish start to lead coming off the bend and was never going to be caught, the 26-year-old clocking a time of 21.88 to finish 0.21 ahead of 100m champion Shelly-Ann Fraser-Pryce.
USA's Carmelita Jeter - silver medallist over 100m - took bronze, with Campbell-Brown fourth.
The USA then enjoyed a one-two in the 110m Hurdles as Aries Merritt stormed to victory in 12.92, the fastest time in the world this year and just 0.01 off the Olympic record set by Liu Xiang in Athens in 2004.
Team-mate Jason Richardson took silver in 13.04, with Jamaica's Hansle Parchment claiming bronze in a new national record of 13.12. World record holder Dayron Robles of Cuba pulled up injured midway through the race.
And a glorious night for the USA was completed when Brittney Reese won gold in the women's Long Jump.
Russia's Natalya Antyukh won the 400m Hurdles, the 31-year-old just holding off USA's Lashinda Demus at the finish.
Antyukh recorded a personal best of 52.70 to finish just 0.06 outside the Olympic record held by 2008 champion Melaine Walker, with Demus setting a season's best of 52.77.
Czech Zuzana Hejnova claimed the bronze medal just ahead of Jamaica's Kaliese Spencer.
Usain Bolt also eased into the 200m final. The 25-year-old looked impressive, effectively winning his semi-final before halfway and almost jogging down the home straight to win in 20.18.
Jamaican team-mate Yohan Blake was the fastest qualifier after also slowing down markedly in winning the opening semi-final in 20.01.
World record-holder Ashton Eaton enjoys a 220-point lead over team-mate Trey Hardee after the first day of the gruelling Decathlon.