US Open Day 5 - three matches to watch

Serena on song: Serena Williams in her US Open second-round victory over Carina Witthoeft

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

Serena Williams (USA x17) v Venus Williams (USA x13)

It's on! The Williams sisters meet for the 30th time, and if the third-round clash is a little earlier than they would like, it's still a fascinating match-up as they headline the night's action on Arthur Ashe Stadium.

Serena, seeking a record-equalling 24th Grand Slam title and her first since the birth of daughter Olympia last September 1, holds a 17-12 career record over her elder sister, whose seven Grand Slam titles include back-to-back US Opens in 2000 and 2001.

Rafael Nadal (ESP x1) v Karen Khachanov (RUS x27)

World number one and defending champion Rafael Nadal has swept into the third round without a hitch, advancing through an abbreviated first round match with Davis Cup teammate David Ferrer then dispatching Canadian Vasek Pospisl in two hours.

That doesn't bode well for Khachanov, a 22-year-old Russian who has lost four career matches against the Spaniard. Khachanov turned in a creditable performance in a 7-6, 6-4 loss to Nadal in Toronto this month, but he doesn't look the man to halt Nadal's bid for an 18th Grand Slam title.

Juan Martin del Potro (ARG x3) v Fernando Verdasco (ESP x31)

Del Potro doesn't mind admitting he's wary of Spanish lefty Verdasco, who is trying to get past the third round of the US Open for the first time since quarter-final runs in 2009 and 2010.

Del Potro, the 2009 US Open champion, withdrew from Toronto and lost in the qarter-finals at Cincinnati, but he's had no trouble dismissing Americans Donald Young and Denis Kudla in the first two rounds and appears primed to handle what could be a punishing baseline battle against Verdasco.