Melbourne, Jan 23 (AFP) Rafael Nadal and Nick Kyrgios reached the Australian Open third round on Thursday after the weather-disrupted tournament faced a new challenge: dirty rain which left courts muddy and unplayable.

After a day of clean-up operations and delays, Australia's Kyrgios fought his way past Frenchman Gilles Simon in four sets and Wimbledon champion Simona Halep stamped her class with a win over Britain's Harriet Dart.

Nadal, a 6-3, 7-6 (7/4), 6-1 winner over Argentina's Federico Delbonis, miscued a shot that hit a ballgirl in the head, but he melted hearts when he apologised and gave her a kiss on the cheek.

"I was so scared for her, honestly," the 19-time Grand Slam winner said.

"The ball was quick and straight on the head. She's a very brave girl."

Kyrgios, increasingly popular with home fans after his fundraising efforts for Australia's bushfire crisis, was cruising at two sets up when he dropped the third set and with it, his composure.

But just when it looked like he would suffer one of his trademark implosions, he rallied for a 6-2, 6-4, 4-6, 7-5 victory.

"I definitely lost my way a little bit... but I decided to refocus," Kyrgios said of his mini-meltdown in the third set.

"I could have gone to a very dark place in the fourth set but I put it away."

Kyrgios and Nadal stay on course for a fourth-round clash and the next instalment in their grudge match after the Aussie hit back at criticism from the "super salty" Spaniard last year.

Their victories followed a day of upheaval caused by the dirty rain, the latest weather problem at a tournament which has contended with bushfire smoke, heavy downpours and strong wind.

Rain mixed with a dust storm coated the Melbourne Park facilities in a fine layer of mud which took hours to clean and made many outside courts temporarily unusable.

- 'I was freaking out' - ========================

As action resumed Alexander Zverev, another man who has a running feud with Kyrgios, showed signs of a return to form as he downed Egor Gerasimov 7-6 (7/5), 6-4, 7-5.

The German seventh seed has been practising up to seven hours a day after a winless ATP Cup and the hard work paid off as he safely reached the third round.

"Definitely much better than the ATP Cup. Now in the third round, I'm very happy about that," said the 22-year-old, who beat Italy's Marco Cecchinato in round one.

However, fifth seed Dominic Thiem had a scare as he was taken to five sets by Australia's 140th-ranked Alex Bolt before recovering his composure to win 6-2, 5-7, 6-7 (5/7), 6-1, 6-2.

"It was all of a sudden a really tight third set that shouldn't happen. That's why I was freaking out inside and also outside today," said the two-time French Open finalist.

A nosebleed was one of Daniil Medvedev's biggest challenges in his win over Spanish qualifier Pedro Martinez, while Gael Monfils, who injured his racquet hand playing computer games before the tournament, downed Ivo Karlovic.

In the women's draw Halep beat Dart 6-2, 6-4, while Belinda Bencic knocked out former French Open champion Jelena Ostapenko -- she was playing despite the sudden death of her father this month.

Two-time Major winner Garbine Muguruza, who scaled Mount Kilimanjaro in the off-season as she searches for a return to form, dispatched home hope Ajla Tomljanovic 6-3, 3-6, 6-3. (AFP)

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