Both players were at 7-under 137, leading a group of five players by one stroke. Derksen shot a 68.
Foster started at the 10th and chipped in for birdie at the 11th. He eagled the 538-yard third hole -- his 12th of the day -- by hitting a 5-wood to 15 feet. He also birdied the difficult 438-yard eighth into the wind with a 5-wood to 12 feet.
Foster earned his only Tour victory at the 2003 Dunhill Championship in South Africa.
Derksen played through some rain and finished before a short suspension.
Martin Erlandsson had an eagle and seven birdies in his tournament-leading 63 on Thursday, but he could only manage one birdie Friday in the tough conditions for a 75.
Colin Montgomerie shot 70, three shots behind the leaders.