Serena Williams became the first player to book her spot in the semi-finals of the WTA Championship, by enjoying a routine victory over world No 1 Victoria Azarenka.
The American won 6-4 6-4 in 97 minutes to claim her third win of the tournament and secure top spot in Group A of this eight-player end-of-season event in Istanbul.
Williams, who is yet to concede a set this week, was broken twice early in the second as she slipped 0-3 behind.
However, she quickly recovered by taking four straight games and sealed victory on her second match point.
The American has been on a hot streak since June, winning the Wimbledon, US Open and Olympic titles and has lost just once in her last 30 competitive matches.
"I have been practising hard and expected to play better. Hopefully as time goes on, I will get better. If not, I hope to play better next year," said Williams.
Azarenka also lost out to Williams in the US Open final and was again left ruing some missed chances against the big-hitting American.
"When I did the right things I was in control much of the time, but I didn't have the consistency today," said Azarenka.
"I had too many opportunities I didn't use. I will be a little bit sad, but I need to bounce back straight away."
Azarenka has now lost her last nine meetings with Williams and Friday's round-robin game against Li Na will decide who joins Serena in the last four.