Iga Swiatek produced a dominant start to her bid for a second WTA Finals title, needing only 61 minutes to dismantle Madison Keys in Riyadh.
Wimbledon champion Swiatek overpowered Australian Open winner Keys 6-1 6-2 in the first match of the season-ending tournament.
The top eight women's players in singles and doubles are competing in Saudi Arabia, where £12m ($15.5m) in prize money will be won.
American fourth seed Amanda Anisimova faces Kazakhstan's sixth seed Elena Rybakina in the other match in the Serena Williams Group on Saturday.
Victory never appeared in doubt for 2023 winner Swiatek after an error-strewn start by Keys, who was contesting her first match since losing in the first round of the US Open in August.