World No.1 Serena Williams regained her winning mojo to leave the US on the cusp of reaching the Hopman Cup final.

Needing to win their tie against the Czech Republic 3-0 in order to qualify for the decider, the US have shot out to a 2-0 lead following singles wins to Williams and John Isner.

Williams overcame the wobbles in the second set to beat Lucie Safarova 6-3 6-7 (1-7) 7-6 (8-6).

Isner then defeated Adam Pavlasek 7-6 (7-4) 6-2 at Perth Arena.

The winner of the mixed doubles will progress to Saturday’s final.

Williams complained of fatigue in her recent matches against Eugenie Bouchard and Flavia Pennetta, but the 33-year-old felt energised on Thursday night.

“I have an ache here and there, but physically I feel like I can run a 10km,” Williams said.

“It feels so much better than I did a couple of days ago. I feel really good now.

“I feel like I’m getting back in the groove. I’m not moving as well as I was last year. But I’m going to try to improve that. I know I can. I’m getting there.

“I needed that win. I was over losing to her in Perth. It feels really good to get through that.”

Williams banged her racquet in frustration during the second set, but her fighting spirit spurred her on during a see-sawing third set.

“I just never give up,” Williams said.

“I have been through so much in my life and on the tennis court, I just keep fighting and just keep doing the best that I can and just keep going for it.”

Isner served 20 aces during his 67-minute demolition of world No.239 Pavlasek.

“The most important thing is we’re still alive,” Isner said.

“We have to come out here and win the mixed doubles now.

“I’ll do the big serves (in the mixed doubles), Serena can do everything else.”