Venus Williams earned a modicum of revenge for her Wimbledon final defeat with a straight-sets victory over Garbine Muguruza, securing her place in the WTA Finals semis.
After a gruelling encounter with Jelena Ostapenko on Tuesday, Williams showed no signs of tiredness as she produced a scintillating display to win 7-5 6-4.
Williams' victory sees her progress from the White Group behind Karolina Pliskova and ends Muguruza's hopes of finishing 2017 as world number one.
It is a second group stage exit in as many years for the Spaniard at the Finals, while Venus remains in contention for a second crown, having won the tournament in 2008.
As in their Centre Court meeting at SW19 in July, there was little between the pair in the early exchanges, both players comfortable on the baseline and able to hold serve.
Williams pressured the Muguruza serve but it was the Wimbledon champion who broke first, converting her second break point with a flat backhand passing shot as Williams came into the net.
Her advantage quickly disappeared, though, as the resilient Williams hit straight back, a pinpoint topspin forehand keeping Muguruza in check.
More great footwork from the 37-year-old enabled her to fizz a cross-court backhand winner but the set remained level until the final game, Muguruza making two costly errors before firing into the net.
That handed Williams the set but completing the win was far from easy as holding serve became tough.
Six breaks in eight games ensured neither was able to gather any momentum, but eventually Williams was able to regain some control and see out the match.
Muguruza's attempts to get her rival moving seemed to be working, and she successfully mixed up her own shots after a pep talk from coach Sam Sumyk.
However, just when it mattered, Williams upped the tempo to take out the last two games and seal her place in the last four and potentially a first title of 2017.
Williams  bt Muguruza  7-5 6-4
Williams - 16/28
Muguruza - 19/26
Williams - 3/7
Muguruza - 3/4
BREAK POINTS WON
Williams - 6/13
Muguruza - 4/10
FIRST SERVE PERCENTAGE
Williams - 60
Muguruza - 70
PERCENTAGE OF POINTS WON ON FIRST/SECOND SERVE
Williams - 67/35
Muguruza - 58/29
Williams - 78
Muguruza - 70