Sitting one shy of a ton, the younger Marsh hit a ball through the offside to bring up his hundred.

During a second run, Shaun grabbed Mitch for a hug but it quickly became apparent the pair had to scramble back to their respective ends to ensure they did not get run out as the ball had not reached the rope.

As it turned out both made it home safely but Smith, sitting with teammates in the stands, was visibly worried something terrible was about to unfold.

Channel Nine showed footage of Smith clapping in delight before yelling in horror at the pair to get back in their crease.

Once the mini crisis had been averted, Smith put his hands on his head and appeared to mouth the words "f*** me" as teammates around laughed hysterically at their animated captain.

Steve Smith's reaction #Ashes pic.twitter.com/C3uIDPwver

"He had a bit of a laugh when we got in the change rooms but it was one of those things that worked out pretty well in the end," Shaun Marsh said. "It was pretty funny.

"That was my fault. Emotions got the better of me and I just wanted to give him a hug. I lost all concept of where the ball was ??? he pushed me off and said: 'You better get to the other end.' Thankfully it all worked out well. I was very happy for him."

One could only imagine Smith's reaction if one of the Marsh brothers were run out while celebrating a proud moment for their family.

"I pretty much went for the high five and Shaun went for the hug and we parked in the middle and had a quick split second of panic and then hugged again after that," Mitch Marsh said.

Social media went into overdrive after footage of Smith was shared around, with many agreeing it was a classic moment of the summer.

After a big celebration to the delight of a healthy SCG crowd, Marsh was dismissed next ball for 101, bowled by an excellent delivery from England seamer Tom Curran.

He will finish the series with 320 runs at an average of 106.66.

The Marsh boys became the first Australian brothers since Steve and Mark Waugh in 2001 to score hundreds in the same innings of a Test.

Combined, the Marsh duo have now scored more centuries (4) than the entire England team (3) across the series.

The elder Marsh looked on track to go past his highest Test score of 182 but was run out for 156 by Mark Stoneman courtesy of a direct hit.

It would be fair to say Shaun and brother Mitch don't hug often. An amicable handshake would normally suffice from time to time.

On Sunday, in oppressive conditions, the pair managed not one, but two emotion-charged hugs as they notched up personal milestones even they would not have thought possible a few months ago.

"It's pretty hard to explain," Mitch Marsh said. "We both had a pretty rough night sleep ??? just to be out there to watch Shaun hit that cover drive and embrace him in the middle when he got his hundred and for him to be out there for mine and help me when I was in the 90s and nervous ??? we'll look back on it in a few years and cherish it."

After a restless night, the older Marsh danced down the pitch and smoked a cover drive through the infield to bring up Test century No.6

That his brother, eight years and three months younger, raised his hands in the air simultaneously shows how much this dig meant to him as well.

It was a hug that meant so much to two cricketers who are not everyone's cup of tea.

As brothers, Mitch and Shaun are tight, more so than the Waugh twins throughout their career and it was fitting they got to share a moment together that very well might not come along again.