Soderling showed no mercy against a clearly fatigued Djokovic at the O2 Arena
By Caroline Cheese
BBC Sport at the O2 Arena
Robin Soderling caused his second upset of the ATP World Tour Finals as he saw off defending champion Novak Djokovic in straight sets.
The Swede prevailed 7-6 (7-5) 6-1 at the O2 Arena to guarantee his place in Saturday's semi-finals.
"I won two matches in straight sets against the world number two and three. I couldn't have asked for anything more," said Soderling, who beat Rafael Nadal in his first Group B match.
The 25-year-old was only invited to the end-of-season finale after Andy Roddick withdrew with injury, but he is the only player yet to drop a set at the tournament.
"I knew for a pretty long time that I would at least be the first alternate here and so I prepared as if I was going to play," he said.
"It doesn't really matter if I'm world number one or first alternate, I still want to win every match. Sometimes I put a bit too much pressure on myself, but I'm trying to work on that."
Djokovic came into the tournament as the form player after a run of 17 wins in 18 matches, but after edging out Nikolay Davydenko in his opening Group B match, the Serb complained of tiredness - and he looked sluggish throughout against Soderling.
"I didn't enjoy playing today's match that's for sure. He was serving really well and all credit to him," said Djokovic, who refused to discuss whether he thought the season should be shorter.
"He won four straight sets and absolutely deserved to qualify for the semi-finals. I think he's the best player so far in the tournament."
Asked whether he could pick himself up in time for his final round-robin match against Nadal, Djokovic responded: "I don't know. I'll see."
After missing out on two break points at 1-1 in the first set, Djokovic's shoulders slumped, and he continually vented his frustrations at his support camp.
At 5-4, the world number three bravely saved three set points, but Soderling would not be denied.
The eighth seed, whose serve was outstanding throughout, appropriately sealed a tight tie-break with a big first serve.
Djokovic spent the changeover muttering to himself, and he could not lift himself in the second set.
Soderling, thriving in his favourite indoor conditions, reeled off six games in a row on his way to a superb win after one hour and 34 minutes.
In Wednesday's opening doubles match, Lukasz Kubot and Oliver Marach saved three match points in a tense Champions tie-break to beat Max Mirnyi and Andy Ram 4-6 6-4 16-14 and move to the top of Group B.