The Czech Republic’s Karolina Pliskova has overcome a scare from the lowly ranked Vicky Duval to level their Hopman Cup tie at 1-1 with the USA.

The plucky Duval, ranked 656 in the world, broke her more fancied opponent’s service game to claim the first set.

However, playing in just her seventh match since August and since being declared cancer free (lymphoma), the 20-year-old America could not keep up the pace with the world No.11 in the second set.

Pliskova won that in just 21 minutes before claiming a 4-6 6-1 6-4 victory.

Duval replaced Serena Williams in the mixed-teams event, the world No.1 nursing a knee injury leading into the Australian Open.

Earlier, the USA’s Jack Sock sent down 25 aces in beating Jiri Vesely in three sets.

The American world No.26 said that it had been a weakness in a disappointing 2015.

So it became a focus of his off-season and he even sent down a few extra balls in short training with good friend and Australia’s world No.30 Nick Kyrgios, in 42 degree Perth heat on Thursday, before his match.

The hard work is paying off, with Sock’s booming serve giving him an edge on Thursday.

After trailing 0-3 to Vesely and then losing the first set, he found some rhythm on his serve to claim the match in three, 4-6 6-2 7-6 (7-3).

His serve gained momentum and the rest of his game followed suit.

“I worked on it a lot in the off season and it feels good right now, so hopefully I can keep it going,” Sock said.

“It’s the only thing that has really felt good all week.

“From the ground today I felt a lot better, so that is a nice confidence booster going into Auckland next week.”

The win was Sock’s first of the round robin tournament, having lost in straight sets to both Australia Gold’s Lleyton Hewitt and Alex Dolgopolov of the Ukraine.

Neither country can make Saturday’s Hopman Cup final after the Ukraine secured top spot in Group A early on Thursday with a win over Australia Gold.