England's Justin Rose and Ian Poulter, and Scotland's Colin Montgomerie and Marc Warren both shot 68 to trail by a shot on 13-under 131. The South African team of Trevor Immelman and Retief Goosen were another shot back after a 69. Three teams were three behind: Denmark (68), Italy (68) and Germany (71).
After a low-scoring day Thursday in the easier better-ball format, scores soared in Friday alternate-shot play at the Mission Hills Golf Club in southern China. Though this is a stroke-play event, match-play rules are in force.
'This is a hard format to play, to keep a rhythm,' said Slocum, who teamed with Weekley for a 61 on Thursday. 'We started the day ahead and we finished the day ahead.'
Added Weekley: 'We played solid today under the conditions. We weren't hitting it our best and sure not putting it our best.'
Weekley and Slocum are surprise leaders. Weekley got the invitation to play only after 13 other American players ranked ahead of him declined. He picked Slocum, his high-school friend from the Florida Panhandle, as partner.
If they win, the prize for each is $800,000.
England had a chance to at least share the lead, but stumbled to its only bogey on the par-3 17th. Poulter ran a 20-foot putt for birdie 7 feet past the hole, and Rose missed to save par.
Poulter and Rose were the pre-tournament favorites -- the only players in the field ranked in the top 20. Rose is No. 8 and Poulter is No. 20.
Scotland drew to within a shot on 18 when Montgomerie dropped a 5-foot birdie putt following Warren's long approach to the green. It was also Montgomerie who helped save par on 17 when his 60-foot chip landed just inches away for Warren to tap in.
South Africa went to 13-under on 17, capitalizing on Goosen's 20-foot birdie putt. But his tee shot on 18 found deep rough, leading to a bogey on the closing hole.
Rose and Poulter were 5-under after 10 holes and briefly held the lead, but lost momentum on the back nine by failing to convert chances on the par-5 holes.
'We certainly got off to a good start and were jelling nicely,' Rose said. 'The back nine just came down to missed opportunities.'
Added Poulter: 'That's just a bit of a shame when we got off to such a flying start.'