Roger Federer says he was "relieved" after his win over Nick Kyrgios on Saturday saw him become the world No. 1 again.

Federer returned to action this week in the Mercedes Cup and is in the final of the event after a 6-7, 6-2, 7-6 win over Kyrgios in the semifinals.

In the process, he earned 150 points, taking him above Rafael Nadal in the ATP rankings to return to the summit for the sixth time in his legendary career.

Now holding a 50-point lead over the Spaniard, Federer could increase it by a further 100 points if he defeats Canada's Milos Raonic in the final on Sunday.

The Swiss ace, who was presented with a plaque by tournament director Edwin Weindorfer after the win, was understandably pleased as he reflected on his achievement and another tough match with Kyrgios that resulted in a tie-break once again.

"I am very happy and very relieved," Federer said, as per the Express. "It was close and it could have gone either way naturally but I am happy that I have got it and got back to world No. 1 next Monday."

"I thought it was the tough match that I expected against Nick. We had so many tie-breaks already I have lost count."

Roger Federer Roger Federer became the world No. 1 for the sixth time in his career. Pictured: Federer of Switzerland poses with tournament director Edwin Weindorfer and ball kids after defeating Nick Kyrgios of Australia and returning to the top position in the ATP global ranking during day 6 of the Mercedes Cup at Tennisclub Weissenhof in Stuttgart, Germany, on June 16, 2018. Photo: Alex Grimm/Getty Images

A win over Raonic now, would represent the ideal start to the grass-court season for Federer as he only gained points because of his round of 16 loss in the event to Tommy Haas last year.

The 36-year-old, who has now conceded two sets in his three wins so far in Stuttgart, Germany, admitted that defeat as well as his recent matches were on his mind but was happy to come through with the victory.

"It has never been easy, even before last match point I thought I had lost the last couple of matches here before match point," Federer explained. "I thought maybe it was going to happen again but when I picked up the half-volley I was so happy."

Federer made history earlier this year in the Rotterdam Open in February when he became the oldest No.1 in tennis history en route to winning the tournament.

He would then lose it to Nadal after losses in the BNP Paribas Open and Miami Open in March before regaining it once again during the clay-court season when the latter was upset by Dominic Thiem in the Madrid Open.

Nadal would then win the Rome Masters to return to the summit before successfully defending his 2,000 points at Roland Garros last week. The latest development in the ATP rankings sees the duo exchange the No. 1 ranking for the fourth time this year as they continue to dominate tennis in their thirties.

Next up for Federer meanwhile, is the Halle Open next week, which he won last year after defeating Alexander Zverev. Following that will be Wimbledon which takes place from July 2-15 as he will be looking to win a ninth crown at SW19.