Emotions run high as Venus Williams downs Heather Watson at Silicon Valley Classic

The typically reserved 38-year-old American broke Watson six times and showed rare flashes of emotion after critical points in the two-hour affair, pumping her fist and shouting

Next up for the seven-time grand slam champion is a showdown with Maria Sakkari of Greece (Getty Images)

Venus Williams found her winning form late and charged to a 6-4 4-6 6-0 win over Britain's Heather Watson at the Silicon Valley Classic on Thursday to punch her ticket to the quarter-finals.

The typically reserved 38-year-old American broke Watson six times and showed rare flashes of emotion after critical points in the two-hour affair, pumping her fist and shouting.

"I'm calm. I can't help how I am," the third-seeded Williams said when asked if she would continue with the new on-court demeanour.

"But I'm definitely fired up inside."

Next up for the seven-time grand slam champion is a showdown with Maria Sakkari of Greece, who strolled to a 6-0 6-1 win over Hungarian Timea Babos earlier in the day.

"I've got to play my best again," said Williams, who has gotten the better of the 23-year-old Sakkari in their two prior meetings.

"It's never easy. I've got to earn it just like tonight so I'll be doing the same thing tomorrow."

Williams is the highest remaining seed in the tournament after defending champion Madison Keys withdrew from the tournament on Thursday prior to her second-round match against Ajla Tomljanovic due to pain in her right wrist.

Watson went down in three sets against the American (Getty Images)

The hard-hitting 23-year-old, runner-up at last year's U.S. Open where she lost to fellow American Sloane Stephens, is ranked 12 in the world and was the second seed in San Jose.

"I am really sorry to have to pull out of the Mubadala Silicon Valley Classic," Keys said. "I have been feeling pain in my wrist over the last couple of days and felt worse today.

Keys was replaced in the draw by Polish lucky loser Magdalena French who fell 6-4 3-6 6-3 to the Australian Tomljanovic to set up a quarter-finals battle with Romanian Mihaela Buzarnescu on Friday.

On Wednesday top-seeded Spaniard Garbine Muguruza withdrew due to a right arm injury.

Reuters