Women's world number one Serena Williams has thanked her improved service game following her fourth-round victory over Madison Keys at Flushing Meadows in New York.

Williams ensured that she remains on course to claim her 22nd Grand Slam title with a 6-3 6-3 victory over Keys, and the 33-year-old believes that her win was in part down to her ability to improve on her serving in the encounter.

"Playing Madison, she's such a good player, and such a powerful player, she knows what to do and knows how to win big matches, so my only chance was to start out fast," Williams revealed on court after the victory.

"I'm so proud that I was able to serve a lot better, and I had to, because Madison has a great serve, so I knew I wouldn't have too many opportunities to break. I was like, 'Serena, it's now or never, you've got to get that serve together.'"

The number one will now face her older sister Venus Williams in the quarter-finals.