MELBOURNE (Reuters) - Seeing world number 50 Mischa Zverev and wildcard Denis Istomin upset the top two players in the world at Melbourne Park shows more players believe they can take shake up the established order and that can only be good for tennis, says Roger Federer.

Zverev's win over world number one Andy Murray on Sunday came on the heels of Istomin's second round victory over six-times champion Novak Djokovic, results which not only surprised Federer but saw the odds of him winning the title shortened.

"I never thought that Mischa Zverev and Denis Istomin would beat those two big guys," said 17-times grand slam champion Federer, who will play Zverev on Rod Laver Arena on Tuesday.

"I guess it's good for tennis that a lot of guys believe stronger now that the top guys are beatable, are vulnerable, especially on a faster court.

"But two huge surprises. No doubt about that."

Federer added that he would practice against a left-hander before the clash with Zverev, who charged Murray at the net and put the Scot off his rhythm from the baseline in their fourth-round encounter.

"Obviously he's on a high right now. He's feeling great," the Swiss said of Zverev. "Probably feels the best he's ever felt on a tennis court.

"It's going to be tough and different and tricky."

Things will not get any easier for Federer should he get past the 29-year-old German, with compatriot Stan Wawrinka or Jo-Wilfried Tsonga waiting for him in the last four.

Tsonga said he was taking it one match at a time.

"It's going to be a tough match," he said of Wawrinka. "I know he's playing really good.

"If I win, another opponent, and if I win, another one, so ... the most important thing for me is to be focused on the next opponent."

In the women's quarters, Venus Williams has the chance to send a positive message to all the veterans at the tournament when the 36-year-old takes on Russia's Anastasia Pavlyuchenkova, 11 years her junior, in the first match on Rod Laver Arena.

Williams could meet her younger sister Serena in a ninth grand slam final if they both get through the draw but she is refusing to think about that scenario just yet.

She would first have to get past Coco Vandeweghe or French Open champion Garbine Muguruza in the last four.

"She's a tricky player," Muguruza said of the 25-year-old Vandeweghe. "She has a lot of power, full shots, serve, everything. She can play very well."

(Writing by Greg Stutchbury in Wellington; Editing by Peter Rutherford)