Having beaten David Ferrer 6-7(3) 6-4 6-1 to claim his second Group A victory at the ATP World Tour Finals, Wawrinka knew a Nadal win later against Czech Tomas Berdych would send him through to the semi-finals along with the Spaniard.
Nadal was already guaranteed his place in the last four but any thoughts of the Mallorcan going through the motions and conserving energy were quickly dispatched as the world number one went about his work with his usual intensity.
Berdych pushed the 27-year-old all the way and thundered through the second set in 29 minutes but Nadal pulled out all the stops to earn a 6-4 1-6 6-3 victory, sealing top spot in the group and avoiding a semi-final clash with Novak Djokovic.
"At the end I play for myself, 200 (ranking points)," Nadal said when asked about the implications of his win for Wawrinka.
"I went on court like as if I am playing for the qualification. I knew (after winning the first set) that I had won the group, but it was important to finish that match the way that I did. I'm very happy for that," added Nadal, who is still in line to earn the $1.9 million for an undefeated champion.
Wawrinka has the daunting task of trying to derail Djokovic's bid to retain the title but the Swiss will not care.
The 28-year-old was the only first-time qualifier at the tournament and with wins against Berdych and Ferrer he has proved he belongs in such exalted company.
Even in losing 7-6 7-6 to Nadal he played superb tennis, actually winning more points than the Spaniard, and Djokovic will be wary of his threat, especially after their epic tussles at the Australian and U.S. Opens this year.
"I can be only really happy with the tournament. It doesn't matter what happens tonight," Wawrinka had told reporters after beating Ferrer, although he admitted he would be glued to his television later rooting for Nadal.
Wawrinka may have felt relaxed enough to change channels and search for a movie when Nadal broke Berdych's serve in the opening game on his way to claiming a first set in which he bludgeoned the Czech all over the spectacular O2 court.
Berdych had lost 14 consecutive matches to Nadal, a morale-sapping sequence dating back to 2006, but he is made of stern stuff these days and was not going to give up on his semi-final hopes without at least unloading some heavy artillery.
He broke Nadal's serve to lead 2-0, held his own to love with some booming 135mph deliveries, then broke to love again.
In the blink of an eye it was 5-0 and Nadal was suddenly rattled by the firepower coming his way, casting anxious glances across to coach 'uncle Toni'.
Nadal, who had won only four points in five games, managed to hold serve to avoid an unthinkable bagel against his name, but Berdych belted down more aces to complete a breathtaking set.
Wawrinka need not have worried though.
Nadal gritted his teeth, battled through his opening service game after being taken to deuce and always kept his nose in front in the third set, cranking up the pressure with some ripping forehands.
Something had to give and inevitably it was Berdych, with two double-faults gifting Nadal the crucial breakthrough in the eighth game before he claimed victory, his 74th of a sensational season, with a searing forehand winner.
Berdych, who still has the Davis Cup final against Djokovic's Serbia to look forward to, neatly summed up the difference on the day.
"Those two double-faults gave him that game," he said. "That game I didn't handle well. That's it. That's the difference between the sixth-ranked player and the number one."
Nadal will now have a day to rest and watch six-times champion Roger Federer try to become the second Swiss player in the semi-finals by beating Juan Martin del Potro in a shoot-out for runners-up spot in Group B.
"If it's Del Potro, if is Federer, both players know very well how to beat me, in this surface especially," said 13-times grand slam champion Nadal, who is assured of finishing the year as world No. 1.
"The only way to win the match of semi-finals is playing my best match, playing fantastic tennis."
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