* Swiss debutant Wawrinka beats Berdych in three sets
* Argentine Del Potro recovers to subdue plucky Gasquet
* Nadal, Djokovic and Federer start campaigns Tuesday
The parade of serial grand slam champions is reserved for Tuesday at the ATP World Tour Finals but the supporting cast provided compelling opening acts with Stanislas Wawrinka and Juan Martin del Potro in the spotlight.
Swiss Wawrinka, the only debutant among the eight players contesting the season's year-ender at London's impressive 02 Arena, proved he belongs with the elite after a 6-3 6-7(0) 6-3 win over Czech powerhouse Tomas Berdych in Group A.
Then, in Monday's evening session, former U.S. Open champion Del Potro came from a set down to eventually subdue French stylist Richard Gasquet 6-7(4) 6-3 7-5 to open Group B.
World number one Rafa Nadal, holder Novak Djokovic and Roger Federer, who have won 36 grand slams between them, all play on the second day when Djokovic's clash with Federer receives top billing after the top seed faces fellow Spaniard David Ferrer.
Wawrinka has long lived in the shadow of compatriot Federer but the 28-year-old has closed to within touching distance of his idol in the world rankings and could surpass him this week.
Not that he will brag about it even if that does happen.
Wawrinka produced some spellbinding strokes as he outclassed the one-dimensional Berdych for much of their two hour 25 minute contest but after clinching victory he was modesty itself when asked about the noisy Swiss support in the 17,000 crowd.
"They wanted Roger, but they had me," Wawrinka, who reached the U.S. Open semi-final this year, told reporters regarding the fans clad in red, many sporting Federer masks.
"It was a really good atmosphere. There were a lot of Swiss people. Yeah, for me it was really, really nice. My first match here was something really special for me."
Only when he lost the second set tiebreak 7-0 did Wawrinka wobble but he quickly went on the attack in the decider, pinning world number six Berdych into the corners with some punishing blows from a single-handed backhand that has purists purring.
The pressure told on Berdych in the fourth game of the final set when he dropped serve and Wawrinka, despite needing some treatment on his thigh, closed out victory in ruthless fashion.
After Wawrinka's stylish win, another of the sport's technicians, world number nine Gasquet, looked on course for victory against towering Argentine Del Potro.
Gasquet, appearing at the season finale for the first time since 2007 after winning three ATP titles this year, dipped into his box of tricks in the first set tiebreak against Del Potro.
A swish of his racket sent a backhand winner darting past the flat-footed Argentine for a 5-3 lead and a rifled forehand return gave him a set point which he clinched with a rolled forehand that angled away into the courtside shadows.
World number five Del Potro, at 25 the youngest in the draw, gradually found his range and the match swung his way in the fourth game of the second set when Gasquet lost a 40-0 lead to drop serve after the Argentine fired two pile-driving forehands.
Gasquet threatened to break back when Del Potro served at 5-3 but, faced with an easy smash which would have put him up 15-40, the Frenchman dumped the ball into the bottom of the net.
Gasquet will probably need to beat either Djokovic or Federer to keep alive his hopes of making the semi-finals, but even in defeat he said there were reasons to be cheerful.
"Yeah, I think I took a little confidence after this match," he told reporters. "Of course, I'm a little bit disappointed because I could have won that match. I was really close to winning.
"Now I need to play two big players again, so it's very difficult. Masters Cup, the level is very high. So every match is a final for me."