Soren Hansen, Raphael Jacquelin and Gregory Havret are tied for fourth place at 6-under-par 66.
Rain fell throughout much of Thursday's opening round and in fact, Houghton Golf Club has been deluged with close to four inches of rain since Monday.
Anders Hansen opened on the 10th tee Thursday and collected only one birdie in his first six holes. He went on a tear around the turn with six birdies in an eight-hole span to get to 7 under par.
The Dane parred his final four holes to remain a part of the tie in first place.
It's almost shocking that Anders Hansen is in this position. Last week at the South African Airways Open, he held the lead with nine holes to play but tallied a pair of sevens on the way in and finished in a tie for ninth place.
'It's nice to play well after last week,' admitted Hansen, who was part of the six-man playoff that Foster won in 2003. 'I wasn't that disappointed because I played really good golf for 65 holes, so I didn't want to dwell on it too much. There are certain things in life you just have to let go.'
Adding to Anders Hansen's woes this week are a lingering right wrist injury and the damp conditions made the problem worse.
'My wrist is bothering me,' said Hansen. 'I don't know what it is, but I'm having treatment for it. It feels like tendinitis. It's also quite damp out there and the ball doesn't go as far as we're used to here.'
Siem also began on the back side but did his damage on his second nine. He drained a curling 20-footer for birdie at the first, then added three more birdies to join Anders Hansen atop the leaderboard.
Siem was grateful for an early tee time Thursday in the straining conditions.
'It was much easier to have the earlier start because the greens are getting bumpy, so it is a little bit unlucky for the guys in the afternoon but I guess it changes around tomorrow,' said Siem.
McLean played the course the way it's intended and collected five birdies before coming to the closing stretch. He failed to use his length to his advantage at the par-5 16th when he left with a par, but he birdied the 17th.
At the 18th, McLean drove into a fairway bunker but nailed a 5-wood from 250 yards out to the front of the green. He got down in two for birdie to round out the threesome in first.
'I think the conditions actually helped me a little bit today,' said the Scotsman. 'I carry the ball a long way but when the fairways are running, a lot of people can get up there too. But if you carry the ball without a lot of run it can be a big help on days like today.'
David Park, Mads Vibe-Hastrup and Johan Edfors are tied for seventh place at 5-under-par 67. Lee Westwood, Alastair Forsyth and Maarten Lafeber are part of a group knotted in 10th place at minus-4.
Foster opened his title defense with an even-par 72 and is part of a logjam in 71st.