In her first tournament back from a knee injury, Williams served for the match twice in the third set and was broken both times, wasting a match point when she hit a backhand long at 5-4. Williams then jumped out to a 5-2 lead in the tiebreaker but missed four consecutive baseline shots to hand Jankovic her second final of the year.
In Saturday's final, the seventh-ranked Jankovic will meet either former No. 1 Ana Ivanovic or Maria Jose Martinez Sanchez of Spain.
Jankovic is aiming for her third title at the Foro Italico after winning in 2007 and 2008.
Jankovic routed Serena's older sister Venus 6-0, 6-1 in the quarterfinals on Friday and became only the seventh player to beat both Williams in the same tournament — joining Aranxta Sanchez Vicario, Steffi Graf, Martina Hingis, Kim Clijsters, Lindsay Davenport and Justine Henin.
Clijsters is the only player to achieve the feat twice.
Jankovic and Williams, the 2002 Rome champ, had a tough time finding their groove at the start, with Williams the first to hold serve to go up 3-2.
Williams then stepped up her attacking game, trying to keep the points short with approach shots to the corners to set up easy put-aways at the net.
Williams won the first set with a big forehand return of serve that Jankovic shanked way off court. However, Williams began to struggle on the longer points - with her fitness apparently an issue — and Jankovic broke at love to go up 4-3 in the second set when Williams committed unforced errors in succession. Williams double-faulted to concede the second set.
With Williams up 5-3 in the third-set tiebreaker, Jankovic served an ace but Williams had her hand up and the chair umpire ruled that the American had called time and Jankovic had to serve again, drawing an argument from the Serb and whistles from the crowd in the Foro Italico's new 10,500-seat stadium, which was only about half full.
Williams finished the match with a whopping 48 unforced errors to Jankovic's 35.
