Luke Guthrie remained the man to catch after the second round of the BMW Masters in Shanghai, extending his lead to four shots on a rollercoaster day at Lake Malaren.
Guthrie was three clear of the field overnight after a flawless opening 65 and quickly extended his advantage with a birdie on the first when play resumed in slightly calmer conditions on Friday.
The 23-year-old's lead fluctuated throughout the round and was finally reduced to a single shot when Simon Dyson birdied the par-five 15th, only for Guthrie to follow with his own birdie minutes later in the group behind.
Dyson then ran up a double-bogey six on the 18th after needing two shots to escape from a greenside bunker, while Guthrie holed from 20ft for a birdie on the 17th.
That took the American five ahead and although he dropped a shot on the last, a second round of 71 gave him a halfway total of eight under par and a great chance of claiming a first European Tour title.
Dyson was among six players sharing second on four under, with Argentina's Ricardo Gonzalez carding the lowest round of the day - a 67 - that featured six birdies and just one bogey on the 18th.
Scott Jamieson's only bogey of the day also came on the 18th as he returned to a 68, while fellow Scot Craig Lee holed out from 188 yards on the 16th for an eagle in his round of 70. Lee had already pulled his putter out of the bag before realising his ball had run back down a slope beyond the pin and into the hole.
Irish Open winner Paul Casey, Thailand's Thongchai Jaidee and Dyson all carded rounds of 70 for the second day in succession to complete the six-way tie, with Spain's Rafael Cabrera-Bello alone in eighth on three under.
Former world number ones Lee Westwood and Rory McIlory are seven shots off the lead on one under, with Race to Dubai leader Henrik Stenson two over after struggling to a 74.
Stenson has a lead of €425,573 over Graeme McDowell on the money list, McDowell also shooting 74 to finish level par.
Padraig Harrington again shot a level par 72 and is tied for 19th, while Paul McGinley (+4) and Shane Lowry (+5) carded one-under par 71s.