It will go down to the closing 800-metre event Tuesday, with Ganna Melnichenko of Ukraine holding a 5,619-5,551 edge over Theisen-Eaton. The Humboldt, Sask., native is known as a better 800 runner.
Theisen-Eaton, who married Olympic and world decathlon champion Ashton Eaton of the United States last month, stood in fourth place Monday after the first day of the seven-discipline event.
She improved her position on Tuesday by finishing fourth in the long jump and 10th in the javelin.
That leaves the newlyweds with the prospect of leaving Moscow with a gold each.
"I watched Ashton win the gold, but I switched the TV off as soon as the result was announced," the 24-year-old Theisen-Eaton, who went to the University of Oregon with Eaton, said on Monday. "I was just trying not to get too excited and to get the needed sleep."
The heptathlon at this year's worlds is wide open because of the absence of Olympic champion Jessica Ennis-Hill and defending champion Tatyana Chernova. Both are out injured.
Drouin advances in high jump
Also on Tuesday, Canada's Derek Drouin qualified for the final of the men's high jump. The Olympic bronze medallist cleared 2.29 metres, the same as the other leaders, to move into Thursday's fiinal.
Fellow Canadian Michael Mason (2.16m) didn't make it out of qualifying.
Farah eases into 5,000 final
Mo Farah easily qualified for Friday's final of the 5,000 in front of a sparse crowd, coasting for the last half lap in the security he would finish in the top five.
"I just wanted to do as little work as possible, freshen my legs up and get through to the final," Farah said.
Russian cakewalk in 20km event
Thinking she had already won the gold medal, Elena Lashmanova came to almost a complete stop.
Inside the stadium with one lap to go in the women's 20-kilometre walk, Lashmanova briefly paused, but soon got moving again and held on to first place to lead a Russian 1-2 on Tuesday in Moscow.
"The judges didn't really explain to us that we should do one lap before the home straight and finish and that is why I stopped," said Lashmanova, the London Olympic champion and world record holder. "But I quickly understood I should keep going."
Lashmanova held off teammate Anisya Kirdyapkina with a final burst of speed to give Russia a sweep of the gold medals in the 20km events after Aleksandr Ivanov won the men's race.
After a dominating the walk through Moscow's morning heat, Lashmanova entered Luzhniki Stadium with a big lead, but then stopped after crossing the line a first time.
With Kirdyapkina closing in fast, a judge urged the 21-year-old Lashmanova to get going again. She did and managed to break the finishing tape on the second go around for gold.
Russians have won seven of the eight women's 20K walks since the event was added to the program at the 1999 worlds in Sevilla.
Russia even had eyes on a sweep Tuesday, but Vera Sokolova was disqualified with about 500 meters to go, leaving Liu Hong of China to take bronze.
"It could have been the three of us on the podium," Lashmanova said.
Because of the walks, the host nation has moved up the medal standings into second place with five overall and two gold, right behind the United States with six overall and three gold.
Yelena Isinbayeva will try to add a third gold for Russia later Tuesday in the pole vault, even though the former world and Olympic champion has not come close to her best this season.
Also on the program on Day 4 of the championships are the finals of the women's 3,000 steeplechase and the men's discus, 800 and 400.