In 2009, Soderling beat four-time champion Rafael Nadal to clear Federer's path to his first French Open title and a career Grand Slam.
On Tuesday, it was Federer's turn for an unexpected defeat. If this keeps up, Soderling's victories won't be upsets anymore.
"I didn't think I played a bad match," Federer said. "He came up with some great tennis. It's much easier to digest this way."
Soderling's next opponent will be No. 15-seeded Tomas Berdych, a first-time Grand Slam semifinalist and the first player from the Czech Republic to reach the Roland Garros semifinals since 1992. Berdych, who beat No. 11 Mikhail Youzhny 6-3, 6-1, 6-2 in the quarterfinals, has won all 15 of his sets in the tournament.
There's the tantalizing prospect that the final Sunday will be Nadal-Soderling rematch. Nadal was to play fellow Spaniard Nicolas Almagro in the quarterfinals Wednesday, and No. 3 Novak Djokovic of Serbia was to face first-time Grand Slam quarterfinalist Jurgen Melzer of Austria.
Federer went in undefeated in 12 matches against Soderling, including a victory in straight sets in last year's final. But Soderling has become more consistent with his thunderous groundstrokes, and he began the tournament with a No. 5 seeding, his highest ever at a Grand Slam event.
"The biggest improvement on the men's tour in the last two years is Robin Soderling's head," said three-time French Open champion Mats Wilander, a fellow Swede. "He believes in himself because he has worked harder than probably anyone to get to where he is."
Federer played some terrific defense, chasing down one shot in a corner near the exit. But opportunities to take the offensive seldom arose.
On match point, the 6-foot-4 Soderling hit a second serve that Federer couldn't put in play. Soderling walked to the net with his fist up as if ready to continue the fight, then pumped it to punctuate the breakthrough against his nemesis - and everyone else's.
"This is a big win, but it's not the final," Soderling said. "I don't want to celebrate too much. I want to focus on the next game."
On the women's side Thursday, No. 5-seeded Elena Dementieva will play No. 17 Francesca Schiavone, the first Italian woman to reach the French Open semifinals since 1954. Dementieva has won six of their 10 matches, including the past three.
Schiavone advanced when she upset third-seeded Caroline Wozniacki of Denmark 6-2, 6-3. Dementieva overcame a sore right thigh to beat fellow Russian Nadia Petrova 2-6, 6-2, 6-0.
In the quarterfinals Wednesday, top-ranked Serena Williams was to play No. 7 Sam Stosur of Australia, and No. 4 Jelena Jankovic of Serbia was to face Russian first-time Grand Slam quarterfinalist Yaroslava Shvedova.
The men's semifinal round will be the first at a Grand Slam tournament without Federer since he lost in the third round of the 2004 French Open to Gustavo Kuerten. That's the last time he lost to anyone other than Nadal at Roland Garros.
Until Tuesday, Federer had seemed unbeatable against Soderling, winning 28 of 30 sets.
"I didn't think about it that much," Soderling said. "You will come closer to winning eventually."
He crossed the threshold after seizing control of the match in the pivotal third set, saving a set point when he won a frantic rally.
Soderling charged forward and hit a slam that Federer scrambled to chase down near the backstop, stretching to whip it back, and Soderling leaped to put away an over-the-shoulder backhand volley.
The stadium shook with a roar, while Federer wore a grim expression.
"That was kind of a hard shot to hit. He hit it well," Federer said.
Following a rain delay of 75 minutes, Soderling won the set to take the lead for good.
Soderling became the first man to beat the defending champion at Roland Garros in back-to-back years since Wilander defeated Yannick Noah in 1984 and Ivan Lendl in 1985.
Federer now begins preparations to bid for a seventh title at Wimbledon.
"You move on to the grass and forget a little bit," he said. "I think I can move away from this rather fast."
