OSAKA, Japan -- No.5 seed Madison Keys of the United States extended her undefeated record against Russia’s Daria Kasatkina on Tuesday, collecting a 6-3, 6-4 victory in the first round of the Toray Pan Pacific Open.

World No.16 Keys improved her head-to-head to 6-0 against the 40th-ranked Russian (12-1 in sets overall) by polishing off a 75-minute victory, to earn her first win at the Premier-level event since her tournament debut in 2013.

"I feel good, I’m always a little bit nervous ahead of my first matches," Keys said, after the match. "It’s always hard to get out there and get all the nerves out, I started a little slow but I was really happy to get the win."

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The American stayed the aggressor throughout the match, with 24 winners, four times Kasatkina's total. Keys also had 30 unforced errors, but Kasatkina fired 17 unforced errors of her own, including seven double faults. Ultimately, Keys was able to convert six of her ten break points to maintain a break lead by the end of each set.

"For me, the key was at the end of the first set and more in second set, when my serve was getting better," Keys concluded. "That was a big part of today’s win."

Keys advances to a second-round encounter with qualifier Zarina Diyas of Kazakhstan, after Diyas knocked out 2016 Olympic champion Monica Puig in a straight-set opener on Monday. Keys won their only prior meeting, which was a straight-set affair in the first round of the 2016 Australian Open.

Kasatkina went up an early break after the first game of the clash, but Keys eventually steered back level, breaking the Russian for 2-2 by firing a backhand winner down the line on break point. The American dropped serve once more in the next game, at love, but obtained parity again at 3-3 after a handful of blistering service returns.

Keys took control of the opener from there, firing two aces to nudge ahead for the first time at 4-3, then using more aggressive tactics while returning to break Kasatkina for a third straight time and earn a 5-3 advantage. Keys had no trouble wrapping up the set, as she held serve at love to put the opening frame in the bank.

Kasatkina ended the four-game winning streak of Keys by holding to open the second set, and the Russian found some stirring winners, reminiscent of the form that brought her into the Top 10 of the WTA Rankings at the end of last year, to garner break points at 2-1. After forcing an error with a deep service return, Kasatkina found herself up 3-1 and serving.

However, after leading 30-0 in the subsequent game, Kasatkina slammed three straight double faults to queue up a break point for Keys. The Russian then slammed a forehand miscue long, and Keys was back on serve. The American then broke Kasatkina at love in the Russian’s next service game to claim a commanding set-and-a-break advantage.

Keys fired a handful of groundstrokes long to allow Kasatkina back on serve at 4-4, but the No.5 seed reclaimed her break lead in a close game to reach 5-4 and serve for the match. Keys did not squander that opportunity, finishing off the tilt after a final Kasatkina return went wide, and the American left the court still undefeated against her opponent.