US Open Day 3 - three matches to watch

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Night time: Serena Williams faces Germany's Carina Witthoeft

Three matches to watch on Wednesday, the third day of the US Open (x denotes seeding):

Serena Williams (USA x17) v Carina Witthoeft (GER)

A potential third-round meeting between Serena and Venus Williams adds spice to the sisters' second-round matches on Wednesday.

Serena, whose 23 Grand Slam titles include six US Opens, went from strength to strength in a first-round win over Magda Linette and she's likely to star again under the floodlights on Arthur Ashe Stadium. Even though she's playing just her seventh tournament since the birth of daughter Olympia last September 1, the US superstar will start as overwhelming favorite in her first meeting against Germany's Witthoeft, the world 101 who claimed her only WTA title at Luxembourg last year.

Camila Giorgi (ITA) v Venus Williams (USA x16)

Seven-time Grand Slam champion Venus Williams negotiated a tricky opening encounter with former US Open winner Svetlana Kuznetsova, and she can expect another tough challenge from hard-hitting Italian Giorgi when they meet on Louis Armstrong Stadium on what promises to be another sweltering afternoon.

"That's her game style," said Venus, who was taken to three sets by Giorgi in the third round of the 2015 Australian Open in their only prior meeting. "I definitely have some pace, too. I'm definitely going to go out there with some weapons."

Like Serena, Venus was reluctant to look ahead to a possible all-Williams third-rounder, which would be their earliest meeting at a Grand Slam in 20 years.

"Obviously the next match, my opponent is not going to roll over," she said. "That's really my focus."

Rafael Nadal (ESP x1) v Vasek Pospisil (CAN)

World number one Nadal eased into his title defense with an abbreviated victory over David Ferrer when his old friend retired in the second set with a calf injury. That means the Spaniard will be well-rested for his second career match with Canada's Pospisil, a straight-sets winner over Lukas Lacko in the first round.

The two have met once previously, with Nadal edging Pospisil 7-6, 6-4 in the second round of the 2015 China Open.

Pospisil, once ranked as high as 25th in the world, has slipped to 88th. He'll need his big serve and booming forehand to be spot-on if he is to provide any challenge at all for 17-time Grand Slam champion Nadal as they close out the night session on Ashe.