The 32-year-old fought hard to defeat Jack Sock 7-5 4-6 6-3 on Saturday (AEDT) in California.
The American had been leading the third and deciding set 3-1, before Millman won the next five games in a row to clinch a hard-fought win after two-and-a-half hours.
Sock, who had been vocal about the line calls throughout the match, appeared less than impressed with the result.
He shared a cold handshake at the net with Millman and seemed to have a few parting words for his Aussie opponent.
Millman was clearly unhappy with what he-d heard and confronted Sock at his bench next to the court.
It was hard to catch everything said in the heated exchange because the on-court announcer was speaking at the same time, but Sock was heard saying: -I didn-t look at you, I smiled at you-.
Millman then responded: -You said ...- before the conversation was again lost in the audio.
-No I didn-t,- Sock was then heard repeating, to which Millman replied: -Yeah you did, mate. And then you proceeded to tell me - (unintelligible) - but yeah, it-s my fault!-
Tennis fans were taken aback by Millman-s strong body language and were convinced something was said to annoy the usually mild-mannered Queenslander.
The umpire even had to get down from his chair to intervene and prevent the exchange from escalating any further.
-A little chatter between the two,- one of the commentators on BeIN Sports said as the tense interaction unfolded.
-That hadn-t been brewing at all but suddenly it has spilt over at the shake of the hands,- another replied.
-We didn-t quite get what Sock said to Millman but you can imagine for Millman to react like that. Sock only has himself to blame.
-He let that match slip through his fingertips. He was never going to give it to him Millman.-
Millman will next play 10th seed and rising Italian star Jannik Sinner on Monday. For more updates, please follow Isports247.net.