Victoria Azarenka of Belarus serves to Heather Watson of Great Britain during their women's singles match at the Sony Ericsson Open tennis tournament in Key Biscayne, Florida, March 25, 2012. [Photo/Agencies]

MIAMI - Venus Williams, playing her first tournament since returning from an autoimmune disease, continued her inspiring comeback at the Sony Ericsson Open on Sunday.

Venus Williams of the US celebrates her victory over Aleksandra Wozniak of Canada at the Sony Ericsson Open tennis tournament in Key Biscayne, Florida March 25, 2012. [Photo/Agencies]

The former world number one followed up her shock win over Wimbledon champion Petra Kvitova with a nailbiting victory over Canadian wildcard Aleksandra Wozniak.

Williams lost the opening set then saved a match point in the third before eventually prevailing 4-6 6-4 7-6 despite making 70 unforced errors.

"I tried not to let anything discourage me today," said Williams, who needed a wild card invitation just to get into the tournament.

"I kept thinking about how I really need this match and I need these points. I keep thinking about the Olympics and I need my points."

Williams, who was diagnosed last year with Sjogren's syndrome, advanced to a fourth round meeting against former French Open champion Ana Ivanovic, who beat Daniela Hantuchova 6-2 7-6.

"You have to be ready emotionally," Williams said. "I think about how miserable I'm gonna feel when I have a loss and I still spend that same time having a win, so I just keep try to digging deeper."

Victoria Azarenka, the current women's world number one, continued to build her winning streak while Rafa Nadal and Andy Murray both sailed into the fourth round.

Azarenka was untroubled beating British wildcard Heather Watson 6-0 6-2. The Australian Open champion is unbeaten in 25 matches this year and on course for her fifth title of the season.

Rafael Nadal of Spain shakes sweat from his head after defeating Radek Stepanek of the Czech Republic in their men's singles match at the Sony Ericsson Open tennis tournament in Key Biscayne, Florida, March 25, 2012. [Photo/Agencies]

Nadal is still chasing his first title this year but made light work of his third round clash with Radek Stepanek, winning 6-2 6-2.

"I'm just happy to be in the fourth round, winning two matches with a positive feeling and with comfortable results," said Nadal, who has made the Sony Ericsson Open final three times but never won.

"I'll just try to keep doing well and try to keep improving my level a little bit."

Murray, the fourth seed in the men's tournament, was given a free ride into the fourth round when his Canadian opponent Milos Raonic withdrew due to an ankle strain he picked up in the second round against French qualifier Arnaud Clement.

Murray, who won in Miami three years ago, will next play Gilles Simon of France.

Radek Stepanek of the Czech Republic throws his towel over his head during an end change after losing a challenge to Rafael Nadal of Spain during their men's singles match at the Sony Ericsson Open tennis tournament in Key Biscayne, Florida, March 25, 2012. [Photo/Agencies]