Serena Williams battled a misbehaving serve and scorching heat but stayed cool under pressure in a 7-5 6-4 win over Czech Barbora Strycova to reach the Australian Open quarter-finals on Monday.

Second seed Williams was broken three times in the opening set at Rod Laver Arena and again when coasting to the finish line in the second but shifted up a gear when it counted to close out the match in one hour and 46 minutes.

"I think she's a really smart player, she can do everything," said Williams of the hard-running Strycova, who caused at shock at last year's tournament by dumping third seed Garbine Muguruza out in the third round.

"It's always good I have something I can improve on and I know I can do better on my serve."

Williams, bidding to win a record 23rd grand slam title in the professional era, has now notched 10 consecutive quarter-final appearances at the majors, dating back to the 2014 U.S. Open.

The 35-year-old great next faces British ninth seed Johanna Konta.

Williams came into the tournament with doubts about her fitness after she had only two tour matches of preparation since the U.S. Open, but she is yet to drop a set at Melbourne Park.

She was left the highest ranked player in the draw after Angelique Kerber's shock elimination on Sunday.

Worryingly for her opponents, Williams has room to improve as she was far from her clinical best against Strycova, racking up 46 unforced errors and winning only six of her 14 break points.

She did not hold serve until her third attempt and was frustrated as 16th seed Strycova saved seven set points in the opening stanza. It took a backhand mishit from Williams to close it out, with her shot barely dribbling over the net.

Williams improved in the second set and raced to a 5-2 lead, as if keen to escape the frying pan of centre court.

She gave Strycova a glimmer of hope, hammering her 45th unforced error into the tramlines to allow the Czech to break back to 5-4.

Like so many times before, however, Williams simply broke back in the next game in a power-hitting assault and Strycova surrendered the match with a volley into the net.