US Open Day 7 - three matches to watch

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

Kaia Kanepi (EST) v Serena Williams (USA x17)

Kaia Kanepi, ranked 44th in the world, took out world number one Simona Halep in the first round. Can the hard-hitting Estonian topple a legend in 23-time Grand Slam winner Williams?

Serena was at her ruthless best in a 6-1, 6-2 second-round demolition of elder sister Venus, calling it the best tennis she's played since returning this year after the birth of daughter Olympia. That's an ominous assessment for anyone in her path to a possible record-equalling 24th major title.

John Isner (USA x11) v Milos Raonic (CAN 25)

Two of the biggest servers in tennis go toe-to-toe for a quarter-final berth. The US Open is the only Grand Slam at which Raonic has never reached the quarter-finals. In addition, the Canadian will be keen to avenge a quarter-final loss to the American at Wimbledon this year.

Isner, ranked 11th in the world, leads their head-to-head 4-1. After reaching a first Grand Slam semi-final at Wimbledon, he'd like nothing better than to take home a first Grand Slam trophy to wife Madison McKinley, who is expecting their first child in late September.

Juan Martin del Potro (ARG x3) v Borna Coric (CRO x20)

It's been years of injury and rebuilding for del Potro since his 2009 US Open triumph, and at a career high of No. 3 in the world the Argentine giant will be a heavy favorite against 21-year-old Coric. The 20th-seeded Croatian, who won his second career title in Halle this year, will find his defensive skills put to the test against del Potro. He'll have to work his way into the rallies to have a chance to spring an upset.