The Finn carded a seven-under-par round of 65 to move into the lead, though Alexander Noren of Sweden is hot on his heels.
Overnight leader Matteo Manassero went round in 71 to drop to third on the leaderboard, four shots behind Ilonen and level with Jonas Blixt.
Ilonen carded an eagle and five birdies as he assumed control.
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- Matteo Manassero