The Continental Europe team grabbed a 3 1/2-1 1/2 lead after the Thursday's four-ball matches.
Ilonen and Olesen combined for seven birdies but the match turned at the 10th hole where Olesen saved par and the Fleetwood-Wood team had a bogey 5. The GBI team won the next hole but Olesen made birdie at the 13th to go 2-up. A conceded hole at 16 cut into the lead but Ilonen and Olesen had pars the rest of the way in to secure the point.
Joost Luiten and Gregory Bourdy teamed for a 2-and-1 win over Jamie Donaldson and David Lynn while Nicolas Colsaerts and Gonzalo Fernandez-Castano beat Marc Warren and Scott Jamison 5-and-3 for the other CI wins.
Paul Lawrie and Stephen Gallagher were 3-and-2 winners over Thomas Bjorn and Miguel Angel Jimenez for a GBI point while CE's Francesco Molinari and Matteo Manassero had an 18th-hole birdie to salvage a halve with Paul Casey and Simon Kahn.
The Seve Trophy is a biennial event in which a team of golfers from Continental Europe takes on a team from Great Britain and Ireland. Continental Europe won the inaugural tournament in 2000 but the GBI team has won the last six, including a 15 1/2-12 1/2 win in 2011.