Australia's swimming golden girl Ariarne Titmus has won her 200m freestyle semi final and will face off with American great Katie Ledecky for the gold medal at the Tokyo Olympics.

The 20-year-old will now swim for her second gold medal in the finals after sensationally pipping Ledecky in a gripping 400m freestyle final on Monday to nab her first gold of the global sporting event.

Titmus hung just behind Hong Kong's Siobhan Bernadette Haughey, who placed second, for the majority of the race but brought it home in the final 50m. 

In the second round of semis on Tuesday morning Ledecky also took out the win and fellow Australian Madison Wilson came fourth but just managed to scrape through into the finals based on her time.

Titmus scored the fastest time in the 200m freestyle semis coming in at one minute and 54.82 seconds. 

Behind her was Haughey at one minute 55.16 seconds, and Ledecky trailing at third with one minute 55.34 seconds.

Ledecky had logged the fastest time during the 200m freestyle heats coming in at one minute 55.28 seconds - seven hours after her 400m freestyle final with Titmus.

Australia's Titmus (1:55.88) was fourth-quickest and rarely approached top gear while compatriot Madi Wilson (1:55.87) was third fastest into Tuesday's semi-finals. 

The Tasmanian-born 20-year-old now based in Queensland has always been rated a genuine gold medal shot in the 200m as well, boasting the world-leading time in the event this year. 

Titmus' 400m freestyle win was Australia's second gold medal of the Tokyo Games, following the women's 4x100m freestyle relay success.

And it delivers Ledecky, regarded as the greatest female swimmer ever, her first defeat in an Olympic final. 

During the 400m final Titmus, in lane three, kept watch on Ledecky in lane four in what became the ultimate match race between the pair. 

The American held the lead for the initial 300m but Titmus was watching her all the way, literally lurking at her heels.

The young Australian ominously surged closer and was just 0.16 seconds behind Ledecky with 100m remaining.

Titmus then reeled in her rival in a perfectly-executed race plan to win by half a body-length in a time of three minutes 56.69 seconds.

The reaction of her coach Dean Boxall after Titmus touched the wall soon went viral with footage showing him ripping off his face mask and running up and down the stands. 

Boxall then grabbed onto a glass barrier and shook it vociferously as he gyrated into the railing.

He then punched the air and screamed at the pool as a startled Japanese Olympic worker nervously tried to encourage him back into his section.  

American star Ledecky, a five-time Olympic champion, is also in the midst of a 1500m campaign.

Soon after her 200m heat, Ledecky's relentless program continued with a 1500m freestyle heat - which she duly won.

Meanwhile, Australian backstroker Kaylee McKeown has taken out the gold and scored herself an Olympic record in the 100m backstroke final. 

Fellow Aussie Emily Seebohm also came in fifth.