Dyson and Siem each shot 5-under-par 65 and are one stroke ahead of Robert Dinwiddie. Four other golfers are at 3-under.
That group includes Jin Jeong, who only got 10 holes in Thursday.
Play was initially delayed 45 minutes while workers repaired damage caused overnight by vandals. The players were later called off the course for 3 1/2 hours because of weather conditions. None of the afternoon groups played more than 11 holes and some played only two holes of the first round.
Dyson played the second nine of the Hilversumsche Golf Club first Thursday and had an eagle at the 484-yard, par-5 12th hole. He had a birdie at 14 and a bogey at 15 before three consecutive birdies allowed him to make the turn at 5-under. A bogey a No. 4 and a birdie at the eighth hole gave him a share of the lead.
Siem had birdies at Nos. 2 and 5 but suffered his lone bogey at the 10th hole. He also had an eagle at 12 and kept pace with Dyson with birdies at 14 and 18.