Swimming Day 4 Roundup: Lilly King and Ryan Murphy Hunt Olympic Repeats

Three of Team USA’s brightest swimming stars chase gold medals on the fourth day of competition from the Tokyo Aquatics Center.

The medal races come thick and fast on Tuesday, with all four finals taking place consecutively, book-ended by a trio of semifinals.

Watch the action LIVE on NBC beginning at 9:30 p.m. ET, or stream the entire session LIVE on NBCOlympics.com and the NBC Sports app.

Getting out to her usual deceptively pedestrian start, Ariarne Titmus produced a sub-29-second final 50 to win the first semifinal in 1:54.82.

Conversely to Titmus, Ledecky went out strong and led at the 150 mark by nearly a full second in semifinal No. 2. Her time, 1:55.34, was third-fastest overall.

Siobhan Haughey of Hong Kong split the pair, qualifying in second in 1:55.16.

Allison Schmitt, swimming in the first semifinal, finished over two seconds behind Titmus. She missed the final in her fourth Olympics.

SEE MORE: Katie Ledecky qualifies for 200m final, will face Titmus

Great Britain's duo of Thomas Dean and Duncan Scott locked out the gold and silver positions, overtaking South Korean flagbearer Hwang Sun-Woo in the final length of the pool.

The margin between the Brits was .04 as Dean won his first Olympic gold medal in 1:44.22.

American Kieran Smith never seriously challenged for the podium, finishing sixth in 1:45.26.

SEE MORE: Brits Dean, Scott go 1-2 in men's 200m freestyle final

In a historic final that included three former Olympic record holders and the two previous world record setters, Australian Kaylee McKeown delivered the second fastest time in history, 57.47, to edge out Canada's Kylie Masse and 19-year-old American Regan Smith.

McKeown missed lowering her own world record by .03, settling for the fifth Olympic record set in the event at these very Games.

Masse got out fastest to the 50-meter, turning ahead of world record pace, but could not hold off the Aussie.

Smith, the Lakeville, Minnesota native, took bronze in 58.05, but was never in touching distance of challenging for gold.

The other American competing in the even, Rhyan White swam well to grab the fourth spot.

SEE MORE: Kaylee McKeown bests Smith, Masse in marquee 100 back final

It was not to be for Ryan Murphy chasing his second 100m backstroke Olympic gold, as Russian athletes Evgeniy Rylov and Kliment Kolesnikov finished ahead.

The women's 100m breaststroke gold medal is staying in the U.S., just not the Lower 48.

Alaska's Lydia Jacoby upset not only fellow American Lilly King, but also South African Tatjana Schoenmaker to win gold.

Chad Le Clos won the first semifinal from lane eight in 1:55.06, but that pace paled in comparison to the mark set in the second semifinal by Hungary's Kristof Milak.

The world record holder went 1:52.22 to affirm his case as the overwhelming favorite for the final.

Americans finished fourth in both heats, but only Gunnar Bentz advanced to the final. Zach Harting missed out.

Kate Douglass and Alex Walsh hope to advance out of their respective semifinals. Walsh swims with defending Olympic champion Katinka Hosszu the first semifinal.