Rafael Nadal claimed revenge against Stan Wawrinka to set up a clash with Andy Murray at the ATP World Tour Finals.
Nadal was making his return to the tournament after missing last year to recover from appendix surgery and he found himself up against the man who beat him at the Paris Masters a fortnight ago.
But this was not the same Wawrinka, with 35 unforced errors in 17 games from the Swiss a major factor in Nadal's 6-3 6-2 victory.
The fifth seed said: "I'm very happy to be back, the last year has been tough.
"I'm very happy to start like this, that's important for my confidence. The last couple of weeks I've been playing well. I have the same motivation, the same spirit to keep improving my tennis."
The round-robin clashes in last year's tournament were nearly all disappointing and so far this edition seems to be heading the same way, with all four singles matches over in straight sets.
Nadal will now take on Murray, who beat David Ferrer 6-4 6-4 in his opening match, on Wednesday.
It was not an auspicious start for Nadal as he lost his opening game to love but, as it turned out, the first nine points of the match went against serve.
Wawrinka was mixing winners with wild shots, and it was one of the latter that cost him in the eighth game before Nadal clinched the set with an ace.
The opening game of the second set seemed to be a turning point as Wawrinka fought off seven break points but Nadal wrested the momentum back by fighting off two of his own in the next game, the second with an exquisite lob that the Spaniard celebrated with a leaping fist pump.
Wawrinka was a picture of frustration and he vented his annoyance at Nadal's slow play to umpire Mohamed Lahyani.
Nadal has undoubtedly had a difficult season but he is ending it with confidence restored after a series of encouraging displays.
Wawrinka continued to spray the ball to all parts of the O2 Arena and Nadal wrapped up victory after an hour and 23 minutes.
