The relay record is in danger of falling again in Wednesday night's final with Australia, Great Britain and Canada all qualifying behind the USA.
To complete the drama, Lilly King of the United States took out the women's 100m breaststroke world record to take the gold.
"I have got the first 150m worked out, I just have to get that last 25m sorted", she said.
"Let's see what comes out in tomorrow's (Wednesday's) final".
Surprisingly, Pellegrini's finishing time was actually.04 seconds slower than Ledecky's winning time in the semifinals. Mireia Belmonte, the Olympic gold medalist in this event and the best Spanish swimmer in history, will be fighting to the wall against the top seed Germany's Franziska Hentke and China's Zhou Yilin.
Winning six golds at the championships would be a record-tying achievement for Ledecky, a record now held by fellow Team USA swimmer Missy Franklin, AP reported.
Ledecky was gracious in a rare moment of defeat, but clearly unhappy with her performance. "I think I'm still learning ... over the years, over the months on how to manage those expectations, but I don't think that had any effect on me today".
Watch the final stages of the 200m event with Pellegrini claiming the gold medial.
"It happens", said Ledecky, who will be a heavy gold medal favorite in her last two events, the 800 free and 4x200 free relay.
Masse, 21, then proved the current depth in women's backstroke with a devastating display.
Efimova almost broke Ruta Meilutyte's 4-year-old record in the semifinals, giving her the prime lane in the middle of the pool.
"I was making sure I was reading the right name and the right time I was lost for words immediately after, I'm still in shock", said a stunned Masse.
Masse was followed by Kathleen Baker of the US and Australia's Emily Seebohm.
"It's awesome, I kinda thought she would do it", said Seebohm, the 2015 world champion, of Masse's record.
"I knew this morning would be challenging to get going".
The finger-wagging American won gold at the Rio Olympics last summer after spurning Efimova and brazenly proclaiming the Russian star had no business being allowed to compete because of doping violations.
The five-time Olympic gold medallist went straight to the warm-down pool after her win in the 1,500m free and turned over around 900m before being pulled out by her coaches to prepare for the victory ceremony and the 200m free semis.
It was Sun's second gold of these championships after his victory in the 400m freestyle final on Sunday.
Le Clos, the 2012 Olympic champion, clocked 1:53.33, with Hungary's Laszlo Cseh, 0.39 seconds behind, taking silver and Japan's Daiya Seto earning bronze at 0.88sec.