Greene (18) ran a personal-best time by 2.19sec when she finished seventh in the 3000m in 9min 21.23sec.
Whyte (18) finished third in his heat of the 400m in 46.62sec. It was a personal-best time and improved on the 46.73sec he ran in Germany last week.
Whyte, who is coached by Brent Ward, could break the Otago senior men's 400m record of 46.57sec in the semifinals. The record was set by Cory Innes in 2006.
Whyte qualified for today's semifinal, where he is ranked 13th of the 24 starters.
She was still in contact with the leaders with 600m to run but could not lift her pace when the African runners upped the pace.
Greene will also contest the 1500m tomorrow.
Greene's performance was the best by an Otago athlete at the world junior championships.
Blair Martin was ninth in the 10,000m in the early 1990s. Max Smith finished ninth in the 1500m at the word youth championships in Hungary in 2001.
Greene, who finished second in the world schools cross-country in the Czech Republic two years ago, has confirmed her ability to rise for the big occasion.
Her coach, Jim Baird, was impressed by the Barcelona run.
"It was a very good performance but I think she could have pushed out a bit earlier," he said.
"She left her run a bit late."
Greene is a member of the New Zealand development squad that is preparing for the Olympic Games in Rio de Janeiro in 2016.
"Barcelona is a learning experience for Becky," Baird said. "I want her to be focused, strong-willed and determined."
Baird believes Greene's performance in the 3000m against the elite African athletes will give her confidence for the 1500m.
"I think Becky is capable of running under 4min 10sec and getting a personal-best time," he said.
Greene's best 1500m time is 4min 18.90sec.
The race was watched by Greene's family, parents Mike and Anita and sister Jamie, who are in Barcelona for the championships.
