Rafael Nadal
Nadal will finish 2013 as world No.1.

Rafael Nadal will finish 2013 as world No.1 after defeating Stanislas Wawrinka 7-6 7-6 to qualify for the semi-final at the ATP World Tour Finals at London's O2 Arena.

Victory means the 11-time grand slam champion will end the year at the rankings' summit for the first time for three years - ending Novak Djokovic's challenge - after a season during which he has won the French and US Open following seven months out injured.

The Spaniard required a tiebreak in the first set after letting slip an early break when serving for the set, but powered through Wawrinka with seemingly fearless aggression.

With Wawrinka faltering, Nadal seemed to be coasting into the last four with a 4-2 lead in the second and despite the Swiss rallying to secure another tiebreak, he missed a set-point chance before the No.1 seed edged through to secure his 12th successive win over the No.7 seed.

The 26 year old Nadal is now guaranteed a place in the semi-final and keeps his hopes alive of claiming his maiden title at the season ending championships.

"I think that after all what happened last year, I felt I deserved to be there at the end of the season," said Nadal.

"This is one of the best things I've done in my career, to come back to No.1 after three seasons. That's difficult in our sport, and after a very important injury.

"That's an emotional thing for me, for sure. In the same time to be able to qualify for the semi-finals is an extra thing. After two matches I can be really focused on the tournament.

"All the support that I had around the world, around the tournaments, every tournament that I played, I had probably more support than ever. All that support was really, really important for me.

Wawrinka, the world No.8 who must beat David Ferrer on Friday to qualify for the semi-final on his world tour finals debut, was angered by Toni Nadal appearing to coach his nephew in the second set, and submitted a complaint to umpire Carlos Ramos.

But the 28 year old says his objection fell on deaf ears.

"We all know, players and umpire, that Toni is always trying to help Rafa," the US Open semifinalist said. "That is normal, that's part of the game but when it's too much, it's too much.

"I didn't agree with the umpire that he didn't tell him something or he didn't give him a second warning just because it was Rafa. Before every point he was trying to coach him. It's nothing against Rafa or Toni, that's in the rules."

In the evening session, Tomas Berdych gave his hopes of qualifying for the ATP World Tour Finals last four a shot in the arm after a 6-4 6-4 win over David Ferrer, who is out after a second straight sets loss in London.

Berdych opened up an advantage after converting his fifth break-point chance of the first set with a crafty backhand volley down the line before clinching the set with a ruthless service game.

Ferrer, who reached the Paris Masters semi-final last week, appeared to be running on empty, but saved two break points at the start of the second set.

However, the double save only delayed the inevitable as Berdych set up three break points in game nine and converted the second as Ferrer found the tramlines.

The Czech world No.6 subsequently served out the match to give his tour finals hopes a lifeline, while Ferrer is out after a run of reaching three successive finals.