Speculation has been rife that the 22-year-old is set to join the west London club in the summer, with reports in Germany suggesting he has alrady agreed personal terms on a move to Stamford Bridge.
The midfielder is expected to sign a five-year contract with Kevin De Bruyne heading in the opposite direction on loan - though Leverkusen are thought to be eyeing a permanent switch.
"Yes, I was in London to negotiate," Voller told Bild. "There is nothing more to add. Nothing is decided yet."
According to Kicker, Voller then travelled to Brussels to meet with representatives of De Bruyne regarding a possible move to BayArena having spent this season on loan at Werder Bremen.
The Leverkusen midfielder has scored 16 goals in 60 games for the club, prompting a promotion to the Germany national team with a return of seven goals in 22 caps.