Federer will top Group B after producing his third dominant display at this year's prestigious end-of-season event at London's O2 Arena.
The world number two's 7-6 (7/5), 6-3 victory over Söderling also means Andy Murray can book his spot in the final four by beating David Ferrer later on Thursday.
Söderling has no hope of qualifying due to the tournament's complex rules involving the percentage of games won by each player.
Federer, bidding to win the title for the fifth time, said, "I'm happy I was able to come out and play a good, tough, solid match against a player who I thought was in good shape. I purposely didn't want to know the calculations before the match.
"Last year, that played on my mind a little bit. I'm happy that all three matches were straight sets wins with no wasted energy. I'm playing real well. I'm through to the semis. So it's all real good right now."
Söderling was left to rue a crucial error that allowed Federer to take the first set.
"At first, I was pretty sure it was going out. But as soon as I let it go, I felt like this one is going much closer to the line than I expected. Of course, it went in. It was a little bit unlucky but it's my fault," he said.
The Tour Finals' round-robin system forces every player to reach for the calculator to work out the various permutations that could see them through.
For Federer at least, the equation was relatively simple. He simply needed to win one set to be guaranteed top spot in the group, while
Söderling's potential path to the last four was rather more headache-inducing.
After a disappointing season, which saw him lose the French Open and Wimbledon titles he won in 2009 as well as the number one ranking, Federer has been in rare form over the last few days in London.
His imperious display against Murray on Tuesday followed an equally
convincing straight sets win over Ferrer and a record of 14 wins from 15 previous meetings with Söderling suggested he would get what he needed to qualify.
It took the six-time Wimbledon champion just three games to secure the
first break as Söderling self-destructed with three successive unforced errors.
However, when Söderling finds his range with his booming groundstrokes, he can be an intimidating customer for even the very best.
Söderling's baseline barrage kept Federer off balance in the eighth game and forced him into some uncharacteristic errors, which allowed the world number four to break back.
Söderling's one success against Federer this year came in the French
Open quarter-finals and he was starting to rattle him in much
the same way he had on the clay at Roland Garros.
However, Federer is a different animal on hard courts and he regained his composure quickly enough to ensure the set went to a tie-break.
A sublime forehand from Federer secured two set-points in the breaker and he converted the second with a blocked backhand that Söderling opted to leave, only to turn and watch in horror as it clipped the line.
That misjudgment was enough to book Federer's semi-final place, but
Söderling was still fighting for his tournament life and he missed a chance to get back in the match when he squandered two break points at the start of the second set.
Although Söderling fought off two break points in a marathon fourth game, Federer was playing with the drive of a man who needed the win far more than he actually did.
He kept coming at Söderling and secured the only break of the second set with a tremendous whipped forehand winner before serving out the match.