Serena Williams was proud of her mental strength, saying she needed to enter "warrior mode" to beat Christina McHale at Wimbledon.

The world number one fought out a 6-7 (7-9) 6-2 6-4 victory over her fellow American on Centre Court in the second round on Friday.

Williams, a 21-time grand-slam champion, acknowledged just how deep she had to dig to stay in the tournament.

"I felt like I was definitely in warrior mode," she told a news conference.

"I was out there playing for my life at that point, trying to play to stay in the tournament."

Williams even found herself a break down in the third set, trailing 2-0, but she responded immediately on her way to winning six of the last eight games.

The 34-year-old said she always knew she could back her mental strength in times of need.

"There were times where I was down and out. I just kept fighting," Williams said.

"That's what I know I can do best. I knew that I could count on that, rely on that."

As for McHale's performance, Williams was impressed, saying: "She played better than I've ever seen her play before.

"She was just hitting line shots from the first point to the last."

Awaiting Williams in the third round is German Annika Beck in what will be their first meeting.