Curtis shot his second straight 67 over the TPC-San Antonio course and his 10-under 134 total put him two shots in front of David Mathis when play was halted by darkness. A weather delay of 2 hours during the afternoon eventually prevented 22 players from finishing the round.
Curtis became the surprise winner of the British Open in 2003 and has won just twice since. He has spent part of this year playing on the European Tour, where he is still exempt due to the British Open victory.
He has made the cut in just one of his other three PGA Tour appearances this season.
Mathis was alone in second at 136 after shooting a 67 while Cameron Tringale was at 7-under 137 after firing a 65. First-round leader Matt Every, who broke the course record with a 63 on Thursday, was at 7-under with two holes to play when darkness fell, was 2-over for his round.
No other player was closer than within six shots of the lead. Blake Adams, Ryan Palmer and Hunter Hass were all at 140.
After the rains left the area, a brisk wind came through and made scoring conditions difficult during the afternoon. It took a score of only 4-over 148 to make the cut and among those missing it was Colt Knost, who contended last week for the title at the RBC Heritage.