That's because even after a 6-7 (3), 6-4, 6-1 victory over David Ferrer, Wawrinka needed a Nadal victory to advance to the semifinals of the Barclays ATP World Tour Finals.
"But even if Berdych wins against Rafa, it's amazing week for me already," Wawrinka said. "I had to win against Tomas Berdych, No. 5, David Ferrer, I lost a tough one against Nadal. At the ATP World Tour Finals, it was quite good level for me. That's the most important.
"Doesn't matter what happens tonight."
Oh, but it did. Wawrinka texted ESPN analyst Darren Cahill during the match and said he was more nervous watching Nadal-Berdych than during any of the matches he played in London.
Wawrinka probably knew in his heart that the No. 1-seeded Nadal -- even though he had already qualified for the semifinals by winning his first two round-robin matches -- wouldn't take the night off. Not when losing in straight sets would have meant losing the Group A lead and facing Novak Djokovic in the semifinals.
Still, Rafa was tested by Berdych before winning 6-4, 1-6, 6-3.
Berdych played like a champion, despite coming in having lost 15 straight matches to Nadal -- and 35 of 37 sets.
Nadal (3-0) will play the winner of Saturday's match between Roger Federer and Juan Martin del Potro in a Sunday semifinal. The No. 7-seeded Wawrinka (2-1) plays Djokovic in the other semifinal. Not bad for Wawrinka's first career appearance at the year-end championships.
Wawrinka saved six of eight break points and converted three of four against Ferrer, who looked sluggish in losing all three of his matches at the O2 Arena. This was his seventh tournament in the past seven weeks.
"I saw at the beginning that he's not really at his best," Wawrinka said. "He's a little bit tired, especially mentally. Every long rally, I had the control of it. For me, the most important it was to fight with myself to keep the ball in, to try to stay with him, to show him that I'm going to play some long rally, that I'm going to try to be aggressive, but not rushing too much."
It was Wawrinka's third win over a top-10 player this year, following a pair of victories over Andy Murray. Ferrer finishes the season with a 60-24 record, the third time in four years he has won 60 matches.
Peya, Soares go through
That's because Paes and Stepanek didn't take care of their business earlier, losing to the defending champions, No. 4 seeds Marcel Granollers and Marc Lopez, 4-6, 7-6 (5), 10-8. That left them hoping the Spanish team of David Marrero and Fernando Verdasco, the No. 6 seeds, who had already clinched a spot in the Group B semifinals, would beat No. 2 Alexander Peya and Bruno Soares in straight sets.
It didn't happen that way. Peya and Soares (2-1) dusted the team of Marrero and Verdasco 6-3, 7-5 and earned a berth in the final four.
Standings, through Day 4
Note: Players in each group meet once each to determine the four semifinalists. Saturday is the last day of round-robin play.
(1) Rafael Nadal 3-0*
(7) Stanislas Wawrinka 2-1*
(5) Tomas Berych 1-2
(3) David Ferrer 0-3
(2) Novak Djokovic 2-0*
(4) Juan Martin del Potro 1-1
(6) Roger Federer 1-1
(8) Richard Gasquet 0-2
* Clinched spot in semifinals
Forward spin: Day 6
They met on the grandest of stages more than four years ago -- and a 20-year-old took down a 15-time Grand Slam champion.
Yes, del Potro was better than Federer that day in the 2009 US Open final, in five captivating sets. It was a remarkable victory, considering del Potro had managed to win only three games when they played earlier that year in the Australian Open quarterfinals.
To date, it's the 25-year-old Argentine's only Grand Slam singles title.
On Saturday afternoon in London, the youngest and oldest players in the singles field will play for a berth in the semifinals. Federer, 32, has a 14-5 head-to-head advantage, but recent history suggests this one might be a push.
No. 4 seed del Potro beat Federer in the Basel final in late October, then Federer, the No. 6 seed, returned the favor last week with a three-set win in the Paris Masters quarterfinals. Federer has reached the year-end semifinals 11 times in the past 12 years.
No. 2 Djokovic meets No. 8 Gasquet in an essentially meaningless match.
In doubles, the No. 1-seeded team of Bob and Mike Bryan must beat No. 8 Mariusz Frystenberg and Marcin Matkowski to clinch a Group A semifinals berth. Both teams enter the first evening match with 1-1 records.