Images: Australian Open Day 3

Comeback queen Justine Henin issued an ominous warning to her rivals at the Australian Open by knocking out Russian fifth seed Elena Dementieva 7-5, 7-6 in the second round on Wednesday.

In a pulsating, high-quality contest, 2004 champion Henin withstood Dementieva's baseline onslaught early, then sealed the match under enormous pressure in the second set to claim her biggest scalp since coming back from retirement after 18 months out of the game.

"I couldn't believe I would relive these kind of emotions," the seven-times Grand Slam champion said in a courtside interview.

"It's only positive things ... The tournament is not over." 

US Open champion Kim Clijsters survived a test from Thailand's Tamarine Tanasugarn to reach the third round of the Australian Open with a 6-3, 6-3 victory on Wednesday.

Clijsters, who appeared to be cruising after breaking Tamarine early in the first set, suddenly met stiff resistance from the 99th-ranked Thai veteran in the second.

The Belgian found herself trailing 3-1 after being broken, but struck back and battled to hold on to her serve after winning a couple of marathon games.

"I haven't played her for a few years. I was actually surprised to see she was still playing," Clijsters said of the 32-year-old Tamarine, a surprise quarter-finalist at Wimbledon in 2008.

US Open champion Juan Martin Del Potro recovered from a set down to advance to the third round of the Australian Open after surviving a marathon battle against American James Blake on Wednesday.

The fourth-seeded Del Potro had been in danger of becoming the highest-ranked men's player to be eliminated before he rallied from a break down in the fifth set to take the match 6-4, 6-7, 5-7, 6-3, 10-8 in four hours, 17 minutes.

The 30-year-old American, who twice had treatment on his right knee that had to be strapped during the match, had held a 2-0 advantage in the final set though the tall Argentine managed to fight back to put the match back on serve.

Del Potro had the opportunity to serve for the match in the 12th game only to be denied by Blake before he finally took the advantage again in the 17th and sealed it with a massive serve that Blake was unable to return on the next game.

It was the second straight-sets win in a row for the in-form Briton, who is looming as a genuine contender to win the first Grand Slam of the year after a mixed display that featured some spectacular shots but also some sloppy errors.

The result seemed a foregone conclusion after the fifth seed made a brilliant start, winning the opening five games with a series of crisp winners.

He broke the Frenchman's serve early in both the second and third sets and despite some lapses of concentration that opened the door for his 57th-ranked opponent, the Scot was able to wrap up a comfortable victory.

Gael Monfils won for the second day running. The 12th seeded Frenchman brushed aside 224th-ranked Croatian Antonio Veic 6-4, 6-4, 6-4.

Monfils reached the fourth round last year, and will try to get back to that point in his third-round match against 33rd-seeded 6ft 9in American John Isner, who ended Irish qualifier Louk Sorensen's dream run, winning 6-3, 7-6(7/4) 7-5.

Defending champion Rafa Nadal secured a place in the third round by thrashing Slovakia's Lukas Lacko 6-2, 6-2, 6-2.

The Spaniard was rarely pushed by 75th-ranked Lacko, who struggled with his serve and amassed 53 unforced errors at Rod Laver Arena.

Lacko saved two of three match points at 5-2 in the third, but hit into the tram-lines on the last to concede the match in less than two hours.

Nadal next plays the winner of the match between Germany's Philipp Kohlschreiber and Wayne Odesnik of the United States.