Australia opener David Warner said his double-century in the Boxing Day test against South Africa was among the finest knocks of his career after coming into the match under pressure to score runs.

Warner retired hurt with cramp after reaching 200 from 254 balls on Tuesday and had tears in his eyes as he limped off the Melbourne Cricket Ground field with the support of a trainer after tea on a sweltering day two.

The 36-year-old received a warm ovation when he returned to the crease on Wednesday but was bowled first ball by a sizzling yorker from Anrich Nortje.

It was Warner's third double-century following 253 against New Zealand in Perth in 2015 and an unbeaten 335 against Pakistan in 2019.

"I was going through (my best knocks) the other day with a couple of the journos (journalists) and that definitely is up there now," Warner told the Seven Network before play on day three.

"To go out there, a lot of pressure, I don't generally feel the pressure, I don't get nervous.

"But walking out here and telling my friends, 'I'm going out to play the way I want to, looking to score and have intent', and to deliver that in a Boxing Day test which is the pinnacle as a kid ... to go out and execute that emphatically was awesome."

Warner had not scored a test hundred in nearly three years before Tuesday and managed only three runs in the series-opening win at the Gabba after averaging 25.50 in the two-test series against West Indies at the start of the home summer.

Warner said his left calf was "absolutely gone" late in his knock on Tuesday and he had an hour-long ice bath to recover before a poor night's sleep.

"When your back's against the wall, you can only look to move forward, that's how I've always been," he said.

"It was emotional, it was hard out there, it was draining.

"The build-up, the articles ... It was a magical moment and so proud to do it in front of my family and friends."