Safina has powerful groundstrokes but in Williams she met an opponent who can hit just as hard and who also had more precision. The two had never played before.
"I had to work very hard because she played at 200 percent,'' Williams said. "It's very exciting to win against a player who has had such a great year.''
Jankovic came back from a break down in each set to edge seventh-seeded Vera Zvonareva for the second time in two weeks, winning 7-6 (8), 7-6 (5). Jankovic is assured of being No. 1 when the new WTA Tour rankings come out Monday.
Safina has won four titles this year and the Russian has risen to a career-high No. 3 in the rankings.
Williams won only one title this year, but it was a major one, at Wimbledon. The American is playing in her first Stuttgart tournament in 10 years and is going for the red Porsche 911 sports car given by the title sponsor to the champion.
"I love fast cars. I really hope to try it as the winner,'' the sixth-seeded Williams said.
If she plays as well as she did against Safina, Williams might have her wish come true. She broke for 5-4 and hit a service winner to take the first set. She then jumped to a 4-0 lead in the second and never looked behind.
She also used her big reach to win points at the net.
"I feel more comfortable at the net now, I have a volley,'' Williams said.
Jankovic also beat Zvonareva on the way to winning the China Open last week and now holds a 5-3 career edge over the Russian. She will take over the top ranking following Serena Williams' second-round loss to Li Na of China.
Jankovic, who lost the U.S. Open final to Serena, is seeking her third title of the year. The Serb leads the tour with 18 quarterfinal appearances this year and is in her 10th semifinal.
At 5-5 in the second set, Jankovic required treatment for a broken toe nail on her left foot.
"I didn't know how to continue, I had no idea how to get through,'' she said. "But I came back with some aces and I fought until the end. I really wanted to finish it in the second set.''
Jankovic was helped when Zvonareva slipped on second match point and the Serb put away a winner.
Earlier, Belarus teenager Victoria Azarenka upset Olympic champion Elena Dementieva 7-6 (6), 3-6, 6-1.
The 19-year-old Azarenka, who saved a set point in the first set, improved to 1-2 against the fourth-seeded Russian.
"I just kept on fighting,'' said Azarenka, who grunted loudly on almost every point. "We had many long rallies. I tried to fight for every ball. I played very well.''
Azarenka will next play another Russian, Nadia Petrova, who beat Li 6-2, 6-3. Petrova, the 2006 champion in Stuttgart, clinched the match with an ace.
Dementieva made too many errors in the third set, allowing Azarenka to race to a 5-0 lead. Dementieva saved three match points to win her first game, but then made two straight errors and put a backhand into the net to lose after nearly three hours.
Azarenka hasn't won a tour title while Dementieva has 10, including at Dubai and the Beijing Olympics. The Russian has returned to her highest ever ranking of No. 4, which she also reached three years ago.
Azarenka is ranked No. 17.
Maniche Suffers Back Injury
Jankovic advances at Pan Pacific Open
Belarus teenager upsets Dementieva in Germany
