Winning the 18th race of the 2015 season for the GT class on Saturday afternoon at Sonoma Raceway was Kevin Estre. Meanwhile, in Round 16 for the GTA and GT Cup classes, Christina Nielsen and Alec Udell were victorious.

After earning pole position in qualifying for the race earlier on Saturday, Renger van der Zande got a clean start in the No. 31 EFFORT Racing Porsche and jumped to an early lead over the No. 9 K-PAX Racing MacLaren 650S GT3 of Estre. Mike Skeen started fourth in the No. 2 CRP Racing Audi R8 LMS Ultra but slotted into third after getting by Michael Lewis in the No. 41 EFFORT Porsche.

Polesitter in the GTA category, Frankie Montecalvo, spun in Turn 4 on the opening lap as drivers went three-wide. Taking over the lead in the No. 7 TRG-AMR Aston Martin Vantage GT3 was Christina Nielsen, ahead of Mike Hedlund, driver of the No. 65 Scuderia Corsa Ferrari 458 GT3 Italia.

Udell, having started second on the GT Cup grid in the No. 17 Global Motorsports Group car grabbed the lead from polesitter Colin Thompson of the No. 11 Porsche of Bucks County entry. On Lap 3, though, Thompson moved back past Udell for the class lead.

On Lap 4, van der Zande led over Estre, Skeen, and the two GT class championship contenders of Olivier Beretta at the wheel of the No. 61 R. Ferri Motorsport Ferrari and Johnny O'Connell of the No. 3 Cadillac Racing car. At that time, Nielsen continued to pace the GTA class race over Hedlund and Martin Fuentes, driver of the No. 07 Scuderia Ferrari. In GT Cup, Thompson remained at the front over Udell, with Sloan Urry of TruSpeed Autosport slotting into third behind them.

On Lap 6, Fuentes got by Hedlund and set after Nielsen in the GTA class. Three laps later, Fuentes went wide into Turn 7a/7b, collecting teammate and GT competitor Duncan Ende of the No. 64 Scuderia Ferrari. Ende spun as a result and Nielsen went wide to avoid it. Race Control issued a drive-thru penalty to Fuentes for his contact with Ende.

Nielsen, Hedlund, and Henrique Cisneros of the No. 30 MOMO NGT Motorsport Ferrari 458 GT3 Italia ran in the top three of GTA. Nielsen remained at the front of the GTA race at Lap 18, but Montecalvo had worked his way up to fourth in was chasing down the three in podium contention.

Estre pulled his McLaren alongside van der Zande through the carousel then took over the lead of the race at 7a/7b on Lap 20. A lap later in GTA, the hard-charging Montecalvo had gotten by both Cisneros and Hedlund to move into second in class behind Nielsen.

Nearing the checkers, GT Cup leader Thompson ran off track at Turn 9 on Lap 23, allowing Udell and Urry to get by for first and second.

At the checkers, Estre beat van der Zande to the line by 2.74 seconds in GT. Finishing fourth and fifth respectively were championship contenders Beretta and O'Connell. For Estre, it's his fourth win of this 2015 season.

Nielsen won her first career World Challenge race in GTA over Montecalvo and Cisneros while Udell won GT Cup over Urry and Thompson.

By turning the fastest laps of the race, Robert Thorne, Montecalvo and Thompson will start on pole in GT, GTA and GT Cup respectively in Race 2 of the weekend at Sonoma on Sunday. Race 2 is set to begin at 1:45 pm ET.

Follow @VAVELSportsCar on Twitter for continued coverage of the Pirelli World Challenge weekend from Sonoma Raceway.

Aaron Durant is the editor of the VAVEL USA Racing section. Follow him on Twitter at @DoubleA291.