STUTTGART - Heavy rain at the Stuttgart Cup on Tuesday threatened to wash away first-round play, with just three matches completed before the skies opened.

There was home disappointment as two-time finalist Philip Kohlschreiber went down to Denis Istomin (UZB) 7-6 (7/2), 7-6 (7/3). The German lost title a bid in 2013 against Italian Fabio Fognini and was beaten two years ago in the final by Dominic Thiem.

It was the first time since 2010 in the national automobile capital that the German had fallen in an opening match here. "I could not work out his serve," the 34-year-old said. "As the match went on, I felt like I was playing worse and worse."

Defeat marked a second straight first-round exit for number 22 Kohlschreiber, who also fell in his French Open start to Croatian Borna Coric.

Istomin, ranked 100th, levelled his Stuttgart record at 1-1 and next faces German Florian Mayer. Hungarian Marton Fucsovics, Geneva title-winner on clay the week before Roland Garros, began his grass campaign by defeating American qualifier Denis Kudla 6-3, 6-4.

French veteran Gilles Simon won a first-time meeting with Italian Matteo Viola, a qualifier, 6-0, 6-3. The early afternoon downpour prevented the start of a match already delayed from Monday, with Canadian teenaged sixth seed Denis Shapovalov due to take on Indian qualifier Prajnesh Gunneswaren.

Roger Federer is set to start in the second round on Wednesday after a bye, taking a 5-0 record -- three of the wins have come on grass -- into his match against German Mischa Zverev. The Swiss can displace Rafael Nadal from the ATP number one ranking for the second times this season if he reaches Sunday's Stuttgart final as he returns to the circuit after skipping the clay campaign for a second straight year.