Sabbatini shot a 5-under 66 at the Cog Hill Golf Club course to tie Steve Marino for the top spot in the third of the four PGA Tour playoff events.
Bo Van Pelt and Marc Leishman were tied for third at 67 and Tiger Woods, who has slipped to second place in the FedEx Cup points race behind Steve Stricker, was among those tied for fifth at 68.
The race for the FedEx Cup and the first prize of $10 million has been reduced to 70 players and only 30 of them will survive to take part in the Tour Championship.
Sabbatini began this week in 36th place and Marino is in 26th place. Sabbatini had hoped to be a member of the International team that will take on the United States next month at the Presidents Cup matches, but Norman left him off the squad when he made his two captain's picks earlier this week.
"Obviously I've struggled over the past couple weeks, couple months, and if I'd been playing well, I'd have something to complain about," Sabbatini said. "You could say I was a little disappointed."
Stricker, a winner last week at the Deutsche Bank Championship, shot an even-par 71 and was tied for 29th place. Heath Slocum, who won the first playoff event and is in third place in the points race, had a 70.