The event, being played at Sonoma, Calif., is the last of the year on the Champions Tour. The trio is tied at 6-under-par 138.
Nielsen (72) led after the first round after a 6-under 66. He made a birdie at No. 18 to move into the tie.
Both Morgan and Roberts carded rounds of 3-under 69. Jay Haas (69), who has won his last two starts on the tour, is tied for fourth place with Craig Stadler (70), Des Smyth (70), Tom Watson (70), Bruce Fleisher (71) and Jerry Pate (72) at 5-under 139.