Warner is third in the overall standings through five events. In the final event of the day, he raced to a personal-best time of 48.20 seconds in the 400 metres.
He wasn't at all surprised with the result.
"I made it a goal to just come out and have fun and I knew the score would just come," said the London, Ont., native. "The score and placing after day one doesn't surprise me," he said. "I have very high expectations of myself."
Warner was fifth heading into the high jump, where he set a personal-best result of 2.05 metres. Earlier in the day, he also set a personal-best 7.53m in the long jump.
Warner has 4,386 points after also competing in the 100m race and shot put.
American Ashton Eaton has the overall lead with 4,661 points, while teammate Trey Hardee trails by 220 points with 4,441.
The 24-year-old Eaton broke Bill Toomey's 44-year-old Olympic record in the decathlon 100m, finishing in 10.35 seconds.
He broke the decathlon world record at the U.S. trials in June when he finished with 9,039 points. His mark eclipsed Roman Sebrle's 11-year-old record by 13 points.
Thursday's final decathlon events are 110m hurdles, discus throw, pole vault, javelin throw and the 1,500m race.
Canada's Cam Levins advances to 5,000m final
Earlier, Canada's Cam Levins advanced to the final of the men's 5,000-metre event on Wednesday at the London Games.
Levins placed eighth overall with a time of 13 minutes, 18.29 seconds, a personal best for the runner from Black Creek, B.C.
"I'm glad to have made it through. I was more confident in the 5[km] than the 10[km]," Levins said in a release. "I knew I just needed to get to the last lap and go. Finishing 11th in the 10[km] provided me with a lot of experience and gave me a big confidence boost that I can race at this level."
Ethopia's Dejen Gebremeskel (13:15.15) and Yenew Alamirew (13:15.39) had the two best times, followed by Kenya's Thomas Pkemei Longosiwa (13:15.41).
The final is set for Saturday.
Canada's Stellingwerff, Sifuentes fail to reach women's 1,500m final
Canadians Hilary Stellingwerff and Nicole Sifuentes did not advance to the final of the women's 1,500-metre final.
Stellingwerff placed sixth in her semifinal with a time of four minutes 5.57 seconds. Sifuentes raced to a time of 4:04.74, placing 11th in her race.
American Jennifer Simpson was the top qualifier for the final, which will take place on Friday. She crossed the line with a time of 3:59.90.
More Canadians were in action Wednesday:
- Jared Connaughton and Aaron Brown were eliminated in the semifinals of the men's 200m.
- Jessica Smith qualified for the semifinals in the women's 800 metres, while Melissa Bishop failed to advance.
- Sultana Frizell and Heather Steacy failed to advance past the preliminary round in the women's hammer throw.
- Curtis Moss finished was eliminated in the qualifying round in men's javelin.