Ian Woosnam, a last-minute replacement, and Bradley Dredge of Wales are alone in second place after a 4-under 68. The French team of Thomas Levet and Raphael Jacquelin paired for a 3-under-par 69 and sole possession of third place.
The World Cup is the fourth and final WGC event of the year. There were 24 teams, Chile withdrew Thursday when Felipe Aguilar injured his hand, and Thursday's action was four-balls or better-ball. On Friday and Sunday, it will be foursomes or alternate-shot, and on Saturday, it's once again four-balls.
The American team of U.S. Open champion Jim Furyk and Justin Leonard opened with a 1-under 71 to share seventh place.
The Ocean Course at Kiawah Island Resort proved to be extremely difficult on Thursday with high winds altering several shots and making for many nervous putts.
The German tandem collected four birdies on the front side to share first place but the twosome parred the first four holes on the second nine. Things would change at the par-3 14th.
Cejka hit his tee ball 18 feet left of the target but Siem, ranked No. 292 in the Official World Golf Ranking, played his drive to three feet. Cejka missed his birdie try but Siem's found every piece of the hole before falling in and giving the Germans a one-shot lead.
At the par-5 16th, Cejka had some trouble and chipped his fourth shot to eight feet. Once again it was Siem to the rescue as the 23-year-old failed to hit the green with his second but chipped to four feet and rolled home the birdie putt to go two clear of the field.
Kiawah was too hard on Thursday for a team to go through without a bogey. At the very difficult par-3 17th, Siem missed the green short by several yards but Cejka played to the right side of the green, 45 feet from the hole. Siem pitched 12 feet from the cup and Cejka lagged his par try to five feet. Neither player was able to save par and the lead was only one.
Neither Cejka nor Siem hit a good drive at 18 but Siem was in better position to save par. Cejka drove into the sand on the right side and barely advanced his ball out of the bunker. Cejka could not get into the hole with bogey but Siem canned a seven-footer for par to hold on to their one-shot lead.
'It was great to have that score and see our name up on the leaderboard,' said Cejka, who finished 60th on the PGA Tour money list in 2003. 'The course has changed a lot since 1997 when I played here and with a wind over 40 miles per hour, it was much tougher. We are both very pleased to lead the tournament after round one.'
Woosnam, who was called to duty during a vacation in Barbados after Phillip Price could not shake the flu, recorded five birdies to erase a 1-over start through three holes.
Woosnam, a winner of this tournament in 1987, seemed happy to represent Wales again, despite being called in from the beach.
'I'd probably be sitting down on the beach or in a bar drinking a nice rum and coke, or going for a siesta,' said Woosnam. 'It didn't take much persuading to come here. It was nice to come and represent your country again.'
Leonard and Furyk, who will be heading to South Africa next week for the Presidents Cup, mixed three bogeys and two birdies over their first 15 holes. They birdied two of their final three holes to join the group in seventh.
Eduardo Romero and Angel Cabrera of Argentina, Carlos Franco and Marco Ruiz of Paraguay and Trevor Immelman and Rory Sabbatini of South Africa all posted rounds of 2-under-par 70 to share fourth place.
The Americans, 1999 British Open winner Paul Lawrie and Alastair Forsyth of Scotland, Mexican duo Alejandro Quiroz and Antonio Maldonado, Korea's K.J. Choi and S.K. Ho, Michael Campbell and David Smail of New Zealand and the Spanish pairing of Miguel Angel Jimenez and Ignacio Garrido are tied for seventh at minus-1.
Japan is the defending champion in 2003. Shigeki Maruyama and Toshi Izawa won the title last year but Hidemichi Tanaka replaced Izawa in 2003. The Japanese team struggled to a 2-over 74 and are tied for 18th.