Singapore: A calm Sloane Stephens picked her shots to perfection to pull clear for a 7-5, 4-6, 6-1 victory over Japan’s Naomi Osaka in a battle between the last two US Open champions at the WTA Finals on Monday.

Both women are making their debuts at the season-ending event, and it was the American who opened Red Group action with a win built on the foundations of solid defence from the baseline and a devastating forehand when she attacked.

“I just competed really well and never gave up,” Stephens said. “I knew I would have to play some really good tennis to beat her, stayed tough and tried to stay as positive as I could and took my opportunities when they presented themselves.

“We played a really tough and competitive match so its always an honour to share the court with somebody that good.”

Simona Halep of Romania maintained the world No. 1 spot in the WTA rankings released on Monday, despite the herniated disc back injury that has forced her to miss the WTA Finals.

Germany’s Angelique Kerber snatched world No. 2 from Caroline Wozniacki of Denmark, dropping her to third, while Osaka of Japan remained world No. 4.

Petra Kvitova of the Czech Republic and Sloane Stephens of the United States both jumped two places to be ranked fifth and sixth, respectively, sending Elina Svitolina of Ukraine and Czech Karolina Pliskova to seventh and eighth.

Russia’s Daria Kasatkina jumped four spots to world No. 10 after claiming the Kremlin Cup title on Saturday.