While the crowd held their breath, fearing the 22-year-old Englishman had done some major damage, Dimitrov hurdled the net and ran to his opponent, eventually helping him hobble off court.
Dimitrov's next match is a quarterfinal against Kyle Edmund, who had a 7-6 (3), 5-7, 6-4 win over Chung Hyeon.
The injury is the latest blow to Britain's prospects at Melbourne Park, with Andy Murray already out and Johanna Konta also struggling with a hip problem.
Millman took some solace from the fact that Dimitrov played two assertive highly-skilled points to save the two match points in the second set tiebreak.
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New York City fire department tweeted that over 200 fire fighters continued to fight the fire at 1547 Commonwealth Ave' in Bronx. There were numerous injuries reported from the morning fire which took over 200 firefighters to control.
Dimitrov will face Australia's Nick Kyrgios in the semi-final after he overcame Ukraine's Alexandr Dolgopolov 1-6 6-3 6-4.
While the Canberran has been heavily criticised for his combustible nature in the past, Dimitrov said it was a clear "strategy" and one that was working for him. "I've seen this too many times".
"I knew the last two games he wasn't at his best so I just had to find a way again", Dimitrov added. That's just me, I guess.
5-7 6-4 in two hours and 38 minutes to reach the quarter-finals of the tournament having impressed in the last round against Canadian teenager Denis Shapovalov.
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This projector also produces incredible sound as the South Korean tech company incorporated two 7W speakers for this device. It is also preloaded with WebOS 3.5 Smart TV interface, that allows users to access popular online streaming services.
Sydneysiders Alex de Minaur and Jordan Thompson have the other two singles wildcards into the Sydney International.
To be honest, I mean, the biggest expectations is always from me.
Karolina Pliskova kept her title defence on track with a 3-6, 7-5, 6-2 win over 2012 victor Kaia Kanepi, setting up a semifinal against Elina Svitolina.
Belarusian qualifier Aliaksandra Sasnovich reached her first tour-level final with a 7-6 (3), 6-4 semifinal win over seventh-seeded Anastasija Sevastova.
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Romney, who at times has been a Trump critic, would enter the race as the front runner, given his own popularity in the state. Romney, a Mormon, has strong ties in the state, helping lead the 2002 Olympic Games in Salt Lake City.