SAN JOSE, CA, USA - In a first-round battle between wildcards at the Mubadala Silicon Valley Classic, Bethanie Mattek-Sands got the better of Venus Williams under the lights.

From a set down, and later, a break down at 1-0, 40-0 down in the final set, Mattek-Sands won six consecutive games to pull off an upset over the seven-time Grand Slam singles champion, 6-7(4), 6-3, 6-1.

"Honestly, I felt pretty good in the first set - but I didn't feel I had her until I shook her hand," revealed the American afterwards. "I know she's always capable of coming back, she doesn't get too intimidated by being down a set or 4-1."

The 34-year-old scored her first singles victory in over a year, dating back to last year's French Open, with the victory, and also tallied her first singles victory on hardcourts since the 2017 Miami Open.

It makes for a positive start to yet another comeback: in February, Mattek-Sands underwent her second knee surgery in three years.

"Part of me was a little crushed to have to stop and go through rehab again," she revealed. "But at the same time I'm glad I had an answer for some of the knee pain I had in Australia.

"I thought I could play through it, I've played through a lot, but at some point you have to listen to your body. Really, if there was no answer, I didn't think I was going to be playing tennis after that. So I got a bit of a second life with this second knee surgery."

Mattek-Sands is a comeback expert by now, and discussed the most crucial aspects of returning to action in detail. "So much of it is mental," she mused.

"I never doubted that I could get my hands back, or the timing, or the rhythm - but I wanted to come back in a different mental space, to play for myself, to play free out there and enjoy it."

"Part of me had to forget all the success I had before. I can't compare my knee to the one I had before because it's never going to feel the same. But I was up for creating a new normal for myself. I'm moving better than ever - I feel I've missed a lot of days in my career and I'm just adding them on to the back end."

The former doubles World No.1 won 27 of the last 31 points in the final set, as she scored victory in the first match between the two in seven years, and her first in five career meetings against the former singles World No.1.

"We've played each other a few times, so we both knew what to expect," said Mattek-Sands. "I knew she was going to come up with some big first serves and I was going to want to neutralise that, so I stood back a bit - I was just mixing it up, mixing my slice up, looking to come to net. That's how I'd play against anybody."

Williams also paid tribute to her opponent's form afterwards: "All credit to her, she came up with her shots," she told press. "Every time I had a game point or a break point it was a winner or a dropshot. I definitely put pressure on her, but shots where I would normally expect a short ball, she hit a winner - she played freely with nothing to lose."

An ultra-aggressive Mattek-Sands kept Williams off balance for the duration of the two hour, 21-minute opening round match by utlizing her doubles expertise, including the dropshot and serve-and-volley play.

Mattek-Sands hit 34 winners to just 24 unforced errors in the match, more than keeping pace with Williams' 31 winners and 34 unforced, and won seven of the 18 break points she created. 

After dropping serve to begin the match, Mattek-Sands rallied from a 4-2 deficit to restore parity, as she finally broke through on her compatriot's serve to level at 4-4, on her seventh overall break point chance.

Despite saving a break point at 5-5 to force Williams to serve for the tiebreak, Mattek-Sands' slow start once there cost her dearly: the former World No.31 dropped the first four points en route to losing the first set. 

The current World No.674 raced out of the gates in the second set, however, as she won the first four games en route to capturing her first set against Williams in over a decade - dating back to the 2009 French Open - to set the stage for the decider.

Mattek-Sands advances to face No.6 seed Carla Suárez Navarro in the second round for a spot in the quarterfinals.