Iga Swiatek won the French Open and US Open this year and is the only major singles champion at the WTA FinalsWorld number one Iga Swiatek beat Daria Kasatkina in straight sets to make a winning start at the WTA Finals.
Poland's Swiatek, 21, defeated the Russian 6-2 6-3 to record her 46th straight-set win this year on the tour.
That is the most main draw straight-set wins recorded in a single WTA season since Serena Williams (66) and Agnieszka Radwanska (46) in 2013.
Meanwhile, French world number six Caroline Garcia, who plays Swiatek next, defeated Coco Gauff 6-4 6-3.
Garcia, 29, is making her return to the singles competition at the season-ending finals after a five-year absence.
Swiatek saved two break points before taking a 3-0 lead against world number eight Kasatkina with a forehand winner in Fort Worth, Texas.
She established the same advantage in taking charge of the second set, facing just one more break point as she recorded her fifth victory of the year over Kasatkina.
The US Open champion is the third player to win 13 consecutive matches against top-10 ranked opponents in a single year since 2000, after Justine Henin (15 in 2007) and Venus Williams (14 in 2001).
"I think I started pretty well and that gave me a lot of confidence," Swiatek said on the Tennis Channel.
"But you have to be really careful and patient against Dasha. I wanted to play in a really solid way but put pressure on my opponent."
The tournament, featuring the eight best players of the year, was moved from usual host city Shenzhen for the second successive year following the WTA's suspension of all tournaments in China amid concern for Chinese tennis player Peng Shuai.
