Defending champion Serena Williams booked her spot in the semi-finals of the WTA Championships as she overcame Petra Kvitova in straight sets on Thursday.
The World No.1 maintained her perfect record in the round-robin stages as she recorded her third consecutive victory of the week, comprehensively beating her Czech opponent 6-2 6-3.
Kvitova emerged as champion in Istanbul two years ago, but struggled to make any impact against Williams who dominated the match having got her nose in front and is now guaranteed to top the red group.
Had Kvitova taken any of the three break points she created in the first game, it might have been a different story, but Williams saved them all.
The American then broke serve immediately and again in the eighth game to clinch the opening set.
Williams' all-round level was simply a cut above with Kvitova's rare winners mostly a temporary distraction from the barrage coming the other way.
The fifth seed made too many errors to put any real pressure on and, when it looked like she might, one of Williams' 11 aces usually helped the defending champion out.
Williams did not lose a match in both her most recent appearances at this tournament in 2009 and 2012 and Thursday's night's win took her run of successive victories to 13 - second only to Martina Navratilova's 21.
Kvitova, who has now failed to beat Williams in all five of their meetings, will still qualify for the semi-finals if she beats Germany's Angelique Kerber on Friday.