But the German tennis star's first thought wasn't of her opponent, Croatia's Donna Vekic. Her head was still getting around the fact she had turned 30 as she thought to herself: "OK, the two is gone right now."

But that realisation she had entered a new decade did not have the same effect on the former world No.1 as it might have had on tennis players in previous eras.

By mid-afternoon she had defeated Vekic in straight sets and her thoughts were positive.

"I'm looking forward to the 30s," Kerber said. "I'm feeling the same [as I did] yesterday. I am feeling healthy. I'm still fit. I still enjoy my tennis. I'm not [thinking] too much about the age, just how I'm feeling. I'm still feeling young."

Such is the changing face of professional tennis, even at a tournament where 15-year-old Ukrainian Marta Kostyuk turned back the clock to become the youngest player to make the third round of the Australian Open in 21 years.

Players are lasting longer on tour as professional preparation helps them through the grind, and the financial incentive to continue remains strong.

The value of experience is also growing as the game evolves. Kerber, who meets Maria Sharapova in the third round on Saturday, says time on the tour can give her an edge on less-seasoned opponents.

"It's changed a little bit in the last [few] years. Experience is also really important in the tennis game right now," Kerber said.

Rafael Nadal and Roger Federer have emphasised how important in the past 12 months, sharing the four grand slams on offer during 2017 despite being in their 30s.

The challenge of taking on up-and-coming players seems to enliven both champions even though Nadal sometimes can't believe what the young bucks try to do on a tennis court.

"We used to wait for the right ball to go for the winner. Now you go for the winner even [when] in not very good positions," Nadal said.

Federer has noticed that shift too, saying racquet technology has made the game faster, points shorter and athleticism a more important part of the game.

"Today, everybody, when they see a mini chance to get on the offensive, they just go for it," Federer said. "It keeps the points short now, too. It's just become extremely athletic in the process."

On Wednesday, 38-year-old Croatian Ivo Karlovic showed that short points can lead to long careers when he became the oldest man to reach the third round of a grand slam in 40 years, with Ken Rosewall's effort at the 1978 Australian Open, when he played aged 44, unlikely to be surpassed.

The big-serving Karlovic outlasted his Japanese opponent Yuichi Sugita to win the fifth set 12-10 to set up a meeting with 33-year-old Italian Andreas Seppi, the two veterans carrying a combined age of 71 into searing temperatures on Friday.

Despite being two sets down and recovering from a bout of gastro the day before Karlovic refused to give up his battle with Seppi and engaged in his second epic battle in three days before losing 9-7 in the fifth set.

Had he won the match, Karlovic would have become the oldest player to reach the final 16 of a grand slam since Jimmy Connors in the 1991 US Open.

Karlovic said the increased professionalism of players allows more to stay on the tour deep into their 30s.

"Everybody is much fitter than the players in the '80s or '90s," Karlovic said. "If you look at them they are really strong overall athletes."

Karlovic is a huge man with a huge serve meaning he doesn't have to chase down as many balls as some, his shoulder working overtime this past week as he served down more than 50 aces in each of his two consecutive five set matches.

But he says it's hard work off the court rather than his serve that has allowed him to continue in grand slams, his daily preparation meaning he was still strong when he passed 30.

"I do everything, gym, weights, running as I try to be a better athlete," Karlovic said. "I don't find feel any weaker than when I was 20. My reaction is a little bit less fast but overall it is OK still."

Karlovic's compatriot Mirjana Lucic-Baroni, was knocked out of the Australian Open in the second round this year aged 35, after making the semi-finals in 2017.

She carries an undiminished enthusiasm for the game and figures that she will remain on the tour for as long as she stays competitive.

"I take a lot of pride being one of the older girls on tour and still being superfit," Lucic-Baroni said.

As a precocious teenager Lucic-Baroni appeared to have the world at her feet as she reached the Wimbledon semi-final in 1999 aged just 17.

She then went missing from the tour for close to a decade, detailing later that the shocking reason for her absence was because she had to flee her father following years of alleged abuse.

"I have missed quite a few years, unfortunately my best years," Lucic-Baroni said.

That gap has given Lucic-Baroni more incentive than most to keep going but she said the advancements in sports science and the amount of physiotherapists on the tour makes the game more conducive to older players who want to continue deep into their 30s.

"[You] spend the time and train like everyone else so you don't look at yourself and think, 'Oh I'm 36, I'm 38'," Lucic-Baroni said.

Lucic-Baroni says resilience comes through planning a sensible schedule using good advice from people with your best interests in mind as much as good physical conditioning.

"Everybody is eager, everybody wants to play a lot, everybody wants to win a lot, everyone wants to get their ranking up and sometimes you can [become] a machine," Lucic-Baroni said.

"It is important to step back and have people around who can say 'OK, time to step back'."

Lucic-Baroni will bring that perspective when she decides to give the tour away but she is not planning to retire just yet.

"As long as I can keep my level high and play big matches and beat big girls and go far in tournaments, why not [play on]," Lucic-Baroni said.

Such an attitude is positive for tennis as it keeps the depth strong and eliminates the shooting stars who then disappeared from trace that were becoming commonplace in the '90s.

The last word should stay with Kerber, the most recent top ranking player to turn 30, who has no doubt one aspect of life remains the key to players staying on the tour longer than they may have in the past.

"I'm enjoying my tennis again," she said. "I think this is the most important thing for me."