Jimmy Walker fired a second round 65 to move to the top of the leaderboard in the WGC-Bridgestone Invitational in Akron.
The 2016 USPGA champion - who revealed earlier this year that he had been diagnosed with Lyme Disease, a bacterial infection spread to humans by infected ticks - carded six birdies and a bogey for a five under par round.
It means the American has a two-shot advantage over Thomas Pieters at the midway stage of the tournament at Firestone Country Club.
Belgium's Pieters fired a 65 himself on the opening day but surrendered the advantage on a rain-affected second day with a round of 70.
Rory McIlroy is also in contention - the Northern Irishman is one of three players a further shot back.
McIlroy, playing his first tournament since splitting from long-time caddie JP Fitzgerald, with whom he won all four of his major championships to date, started strongly with a birdie on the second hole.
Although McIlroy bogeyed the ninth following a wild drive, he gained another shot at the 16th for a round of 69 which leaves him on four under par alongside America's Zach Johnson and Japan's Hideki Matsuyama.
Jordan Spieth, seeking his third win in succession after victories in the Travelers Championship and Open Championship, is one of six players on three under par after a round of 70.
Russell Knox is also three under - the 32-year-old Scot held a two-shot lead before the first suspension in play because of poor weather.
England's Tommy Fleetwood, one of 12 European Tour members making their debuts in Akron, is on two under after a round of 68.
Paul Casey (69) and Ross Fisher (72) are a shot further back. Justin Rose (69) is one over and Tyrrell Hatton and Matthew Fitzpatrick are three and four over respectively after second round 71s.
Scotland's Paul Lawrie slipped down the leaderboard after a 74 left him on three over, England's Andy Sullivan shot 75 to lie on seven over, two ahead of Chris Wood (74) and three ahead of Lee Westwood (78) and Danny Willett (74).