The former world No. 3 also had an injury-riddled 2017 campaign, including a seven-week absence due to wrist surgery that cost him a chance to play at that season's U.S. Open.
Should it be Djokovic, it would make the Serb the first player to come from outside the top 20-he was at No22 in June-to reach No1 in a single season since 2000.
"So I really enjoy tennis at the moment and enjoy competing, because obviously when you're winning that many matches you have a lot of confidence".
That went the distance too but this was a different story for Isner, who won early points against the Kazakh's serve to take the match.
Federer will play Canada's Milos Raonic on Wednesday after receiving a bye in the first round.
The 31-year-old though outlasted Nadal in an epic Wimbledon semi-final before sweeping Kevin Anderson aside to claim his first Grand Slam since the 2016 French Open. "Something that I was not able to do this year, playing only nine events and retiring in two".
Cilic, the fifth seed, secured his quarter-final ticket with a 7-6 (7/5), 6-4 victory over Grigor Dimitrov.
Pakistan delays release of Christian woman after blasphemy acquittal
To which Bibi responded: "I believe in my religion and in Jesus Christ, who died on the cross for the sins of mankind". However, it was not clear where she will go as several countries, including Canada, have offered asylum to her.
"But in the past few days, I started to feel a little bit (of an) abdominal (injury), especially when I was serving".
"I'm happy to play tomorrow".
A fresh Federer made the most of his rest to storm into the lead against the wasteful Fognini, who could not subsequently capitalise when his opponent's performance levels dipped at the end of either set.
Sixth-seeded Dominic Thiem won 6-4, 6-2 against Gilles Simon, beating the Frenchman for the eighth time in 10 meetings.
John Isner, however, is set to miss the Finals after losing to Karen Khachanov in a tussle featuring two tie-breaks in nearly two and a half hours.
Soon after that he split with coach Andre Agassi, then removed Radek Stepanek from his team as his career continued to spiral downwards into the abyss.
Germany's Alexander Zverev triumphed 6-4, 6-4 against American Frances Tiafoe in a topsy-turvy encounter.
Djokovic can regain top spot from Rafael Nadal this week if he betters the Spaniard's performance in the French capital, where the Serb is a four-time victor of the indoor tournament.