Boardman, who recently shattered the one-hour record with 56.3759 kms, will be favourite in the 70-km race around the lake of Madine, in Eastern France.
But Spain's Olano, who beat him into second place in the time trial at the Atlanta Olympics, and Denmark's Riis are among the best specialists not currently taking part in the Tour of Spain.
In the women's race, Russian time trial olympic champion Zulfia Zabirova tops the bill.
Britain's Boardman, who broke the world hour record two weeks ago with 56.375km, is hoping to continue his unblemished season till the world time-trial championship in Lugano, Switzerland in October.
Boardman has already won the Eddy-Merckx grand prix and the Telekom grand prix, but the Nations GP is the most famous and more prestigious event.
His main opposition will come from Tour de France winner Bjarne Riis of Denmark, Belgium's Johan Museeuw, Francis Moreau of France and world road race champion Abraham Olano of Spain.
The world one-hour record holder left Tour de France winner Bjarne Riis trailing by four minutes 51 seconds in the 70 km time trial held around the lake of Madine in eastern France.
Spanish road world champion Abraham Olano, who beat Boardman into third place in the time trial at the Atlanta Olympics, was third, 5:11 behind.
The race was the main time trial to elude Boardman, who this season has won the Grand Prix Eddy Merckx and the Grand Prix Telekom. On his first attempt in 1994 he lost to Switzerland's Tony Rominger.
This year, he was in a class of his own, even though some of the world's leading specialists, including Rominger and Miguel Indurain, were involved in the Tour of Spain.
Boardman had an impressive lead of nearly two minutes halfway through. For good measure he overtook Riis and Olano, who had started four and two minutes respectively before him, and clocked 1:25:39.
1. Chris Boardman (Gbr) 1.25.39 2. Bjarne Riis (Den) + 4.51 3. Abraham Olano (Spa) + 5.11 4. Christophe Moreau (Fra) + 5.27 5. Vjatjeslav Ekimov (Rus) + 5.54 6. Johan Museeuw (Bel) + 6.26 7. Christophe Bassons (Fra) + 6.55 8. Bert Roesems (Bel) + 7.37 9. Pascal Lance (Fra) + 7.37 10. Erik Dekker (Ned) + 8.24 11. Francis Moreau (Fra) + 8.27 12. Renaud Boxus (Bel) +10.33 13. Marek Lesniewski (Pol) +12.35 14. Eddy Seigneur (Fra) +12.41 15. Harel (Fra) +13.16 16. Daniel Van Elven (Ned) +14.48
1. Jeannie Longo (Fra) 50.02 2. Hanka Kupfernagel (Ger) + 1.14 3. Natalia Boubentchikova (Rus) + 1.57 4. Catherine Marsal (Fra) + 2.04 5. Sophie Swaertvaeger (Fra) + 3.01 6. Diana Rast (Sch) + 3.02 7. Svetlana Samokhvalova (Rus) + 3.03 8. Jolanta Polikeviciute (Lit) + 4.10 9. Karin Mobes (Sch) + 4.14 10. Rasa Polikeviciute (Lit) + 4.26--Roger ___________________________________________________________ Roger Thomas Editorial & Research Services e-mail: roger@alcala.demon.co.uk Phone/fax modem (on request): (44)(0)1582 454126 Home page: http://www.alcala.demon.co.uk/ ___________________________________________________________